E2 Transition Metal Chemistry Synthesis and Analysis of a Cobalt Coordination Complex Some of the most interesting area in inorganic chemistry has centered on the preparation and properties of coordination compounds. These compounds, sometimes called complexes, are salts that contain complex ions. A complex ion contains a central metal atom to which are bonded small molecules or ions, called ligands. When the metal atom is a transition metal the complex ion is usually colored (resulting from the absorption of visible light) and often contains unpaired electrons giving rise to their paramagnetic properties. The compound we will prepare contains cobalt ion, ammonia, and chloride ions. Its formula is of the form Co x (NH 3 ) y Cl z, where x, y, and z are integers from 1-6. After we complete the synthesis of this compound we will systematically analyze for each component of the compound. The experiment is done over four laboratory periods. Complete advanced study assignment (found at the end of the report form) for each part of the experiment before coming to lab and hand it in before lab starts! Day 1-Part 1. Synthesis of Co x (NH 3 ) y Cl z Reagents CoCl 2 6H 2 O Co x (NH 3 ) y Cl z 1. Two students (partners) will work together and share data). Set up in the fume hood a clean dry 125-mL Erlenmeyer flask and place it on a ceramic hot plate with a small magnetic stirrer bar. 2. In the balance room place a plastic boat on a centigram balance and tare (set to 0.00 g) and mass out approximately (~) 5 g (±0.1g) of ammonium chloride. Transfer all of this salt to the Erlenmeyer flask. 3. All concentrated reagents are located in fume hood by the door. Use calibration marks on a 50 ml beaker and obtain ~ 30 ml of 15 M aqueous ammonia, NH 3. Cover the 50 ml beaker with a small watchglass and go back to your reaction flask. CAUTION: This is a concentrated alkaline reagent, with a strong pungent odor that can knock you unconscious if inhaled directly. 4. While under the fume hood, remove the watchglass and carefully add 30 ml of 15 M NH 3 to the Erlenmeyer flask. You can do this with your plastic pipet if you are hesitant in pouring from the 50 ml beaker. Turn on the magnetic stirrer (left knob on your stirrer/hot plate, DO NOT HEAT) and allow the NH 4 Cl to completely dissolve in the basic solution. {The combination of NH 4 Cl and NH 3 (aq) guarantees a large excess of the ligand, NH 3. It is important that you follow direction and obtain the following reagents systematically!} 5. Go tare a plastic boat and mass out 10 g (to the nearest ± 0.01 g) of finely divided CoCl 2 6H 2 O(s). Record this mass in your notebook. Your percent yield will be based on this reactant. 6. While stirring the ammonium chloride-ammonia solution add the CoCl 2 6H 2 O(s) in small portions to your reaction flask. This can be accomplished simply by holding the plastic boat near the mouth of your reaction flask. Use your Ni-spatula and spoon or brush a small amount of the purple solid into the reaction mixture. Don t add too fast and allow the solid to dissolve before adding more. Continue adding in small portions until all has been added.
Co SACC Procedure page 2 7. Take your 50 ml beaker and watchglass back to the fume hood containing the hydrogen peroxide, H 2 O 2, reagent. Dispense ~ 10 ml of 30% H 2 O 2 into your 50 ml beaker and cover with watchglass. {No need to rinse beaker since by this time all of the NH 3 you obtained has evaporated.} 8. Back at your reaction flask use your plastic pipet and SLOWLY add in small portions down the inside wall of your reaction flask all of the 30% H 2 O 2 to the brown slurry. CAUTION: 30% hydrogen peroxide is a strong oxidizing agent that can cause severe burns and bleaching of the skin and clothing. Use gloves when handling it. Avoid excessive effervescence (gas evolution) in this very exothermic reaction. {If the addition is SLOW and the reaction still shows excessive effervescence, turn off the magnetic stirrer momentarily or remove the reaction flask from the stirrer.} Continue adding the hydrogen peroxide until all has been added. The initial Co 2+ ion is oxidized to Co 3+ ion during the addition of hydrogen peroxide and will serve as the metal center of the complex ion. 9. Use your 50 ml beaker and watchglass to obtain ~30 ml of 12 M HCl solution. Cover the 50 ml beaker with the watchglass while transporting it back to your reaction vessel. With the use of your plastic pipet again SLOWLY add in small portions down the inside wall of your reaction flask the concentrated 12 M HCl. CAUTION: Concentrated HCl is a strong, fuming acid that will cause severe burns and breathing difficulties, so handle with care! The white cloud that forms is solid ammonium chloride, formed from the acid-base neutralization reaction, and is dispersed in air. After the access NH 3 has been neutralized the white cloud will no longer form upon addition of HCl. Continue adding the HCl in small portions (aliquots) until all has been added. 10. When the addition of the acid has been completed, the reaction may be removed from the fume hood along with the hot plate/stirrer and taken to your benchtop. Use the hot plate/stirrer; place the reaction solution in a water bath containing ~75 ml of H 2 O in 250 or 400 ml beaker and heat the solution to approximately 70 C with occasional stirring (leave stirrer on). For best results keep the temperature between 65 C and 75 C for ~15-20 minutes after obtaining water bath temperature. 11. Remove reaction flask from water bath, add ~25 ml deionized water, (don t forget to remove your magnetic stirrer bar with magnetic wand!) and allow the solution to cool to ~10 C in an ice-water bath (a 400 ml beaker containing ~75 ml of water plus ice). Precipitation of the purple product should continue. Cool ~ 20 ml of deionized (dh 2 O) water in your ice-water bath after removing reaction mixture. 12. Collect the product by filtration through a Buchner funnel. Seat a filter paper (designed to fit the specific Buchner funnel you are using) by running a little distilled water through the Buchner funnel with the vacuum on, as demonstrate by your lab instructor. A single circular #1 filter paper should fit over the holes of the Buchner funnel. Remove your reaction vessel from the ice-bath swirl the contents well until the precipitate is suspended in the solution and filter continuously into the Buchner funnel. Wash the product three times with a total of ~15 ml of cold deionized water and at least twice (smaller portions better) with a total of ~15 ml ice cold 95% ethanol. Allow the aspirator to draw air through the precipitate for 5-10 minutes to allow the product to dry further. Use your nickel spatula cut the cake of precipitate around its edge and transfer the product to a LARGE watch glass. Cut and spread the product out with your spatula. The consistency of your product should be like dry sand. The product should not stick to the spatula. Loosely cover with a paper towel, and allow the product to dry until the following laboratory period.
Co SACC Procedure page 3 Day 2-Part 2. Gravimetric analysis of Chloride in Co x (NH 3 ) y Cl z Here we will analyze for chloride in your purple product. We would also like to determine how many chloride ions are bound to the metal as ligands (part of the metal coordination sphere) and how many serve as counter ions that are not covalently attached to the metal. But before we do the experiment a little background may be necessary. As discussed in lecture Alfred Werner won the Nobel Prize in Chemistry 1913 for proposing the structure of transition metal coordination complexes. The compound Co(NH 3 ) 4 Cl 3 exists in two forms visually distinguishable by their green and purple color. The number of chlorides and their location was chemically confirmed by chloride analysis, as in this experiment, using silver nitrate. Most transition metal coordination complexes are salts, made up of cations and anions. If we dissolve a fixed amount of the green compound and add an excess of AgNO 3 we can capture all the chloride ions that are not covalently attached to the metal. Energy in the form of heat is required to break chemical bonds! Therefore to obtain all the chloride present in the green compound heat is required. So consider the following: xs AgNO 3, no heat 1 mole green complex(aq) 1 mole AgCl xs AgNO 3, heat 1 mole green complex(aq) 3 mole AgCl Interpretation: %Cl(heated)/%Cl(unheated) = moles AgCl(heated)/moles AgCl(unheated) =3/1 The total number of moles of chlorides present in one mole of the compound = 3 (from heated sample) The total number of moles of free chlorides (as counter anions) present in one mole of the compound = 1 (from unheated sample) This result means that there are 2 Cl-ligands attached to the metal (2 Cl - and 4 NH 3 ) and that the complex ion is geometrically arranged in the form of an octahedron; [Co(NH 3 ) 4 Cl 2 ] + Cl - Now for your analysis! 1. First thing, go to the balance room and mass the dry purple product that you synthesized to the nearest ± 0.01 g. Record this value in your notebook as the actual yield! (Partners share this data!) {First you will find how many chlorides are not attached to the metal by dissolving the purple product in water and collecting all the free chloride ions with AgNO 3 (unheated sample!), then you will repeat steps 2 and 3 skip to step 6 where you will break any and all metal-chloride bonds and collect all the chloride in the sample with AgNO 3 (heated sample!).} Chloride Analysis Unheated Sample 2. Each member of the team now works individually, the data obtained will be shared and this will account for two (duplicate) determinations! Using an analytical balance, mass out a sample of the purple product in a weighing boat between 0.45 0.55 g and record this mass to the nearest 0.1 mg (±0.0001g). Transfer this to a 250 ml beaker. 3. Get a magnetic stirrer/heater hot plate from the fume hood or cart, place the 250 ml beaker on the hot plate. Add ~150 ml dh 2 O and a magnetic stirring bar (~ 1 in long). Stir to completely dissolve
Co SACC Procedure page 4 compound. Acidify this solution by adding ~1-1.5 ml of 6M nitric acid (HNO 3 ) and continue stirring. 4. Obtain ~30mL of 0.25 M AgNO 3 solution and add it in small quantities to your reaction mixture with your plastic pipet. Stir reaction mixture for about 5 minutes after you have completed the AgNO 3 addition. While the mixture is stirring obtain a clean, dry fritted glass crucible from the oven or your instructor. Use a pair of crucible tongs and a wire gauze held underneath the crucible for support as you transport it to your desk (this may have been demonstrated by your instructor). When cooled to room temperature mass the glass crucible on the analytical balance to ±0.0001 g. Record this mass in your notebook {mass crucible 1(unheated)}. 5. Cool reaction mixture to ~10 C in an ice-water bath (100 ml of ice/h 2 O in 400 ml beaker). Also cool about 15 ml of 0.01 M HNO 3 for washing. 6. Set up glass crucible, neoprene adapter, filter flask apparatus and connect to aspirator. Turn aspirator on full and check for vacuum leaks and for rate of flow. (If water does not come through fast enough, check with instructor, you may have an old glass crucible and needs to be replaced.) Filter solution through glass crucible first, and using your squeeze bottle squirt small portions of water behind precipitate and rinse all of it into the glass crucible. 7. Wash the precipitate with ~15 ml of cold 0.01 M HNO 3, in small portions of 4-5 ml, until the washings (1-3 drops) collected in a small test tube gives no turbidity when tested with a drop of 3M HCl. If the drops in your test tube are turbid (cloudy) continue washing with 0.01 M HNO 3 to free the precipitate collected from excess silver ions. The turbidity can be tested by breaking the vacuum above the neoprene adapter, removing the glass crucible and transferring any drops of the solution adhering near the bottom of the crucible to a small test tube, as demonstrated by your instructor. 8. When the precipitate has been collected wipe the outside of your glass crucible with moist kim-wipe paper towel and dry to remove oil and fingerprints. Place the glass crucible in a small, dry 50 ml beaker insert a piece of paper with your NAME, unheated sample, and locker number between the crucible and the beaker and place it in the container provided for oven drying. This completes the chloride analysis for the unheated sample. Chloride Analysis Heated Sample 9. Use an analytical balance as before, mass out a sample of the purple product in a weighing boat between 0.45 0.55 g and record this mass to the nearest 0.1 mg (±0.0001g). Transfer this to a 250 ml beaker. 10. Place the 250 ml beaker on your hot plate. Add ~150 ml dh 2 O and a magnetic stirring bar (~ 1 in long). Stir to completely dissolve compound. Acidify this solution by adding ~1-1.5 ml of 6M nitric acid (HNO 3 ) and continue stirring. 11. Obtain ~30mL of 0.25 M AgNO 3 solution and add it in small quantities to your reaction mixture with your plastic pipet while stirring. When all of the AgNO 3 solution has been added, turn the hot plate heater ON and heat the suspension with stirring until the solution boils. Continue boiling the mixture with stirring for ~20 minutes. This digestions process aids the completeness of precipitation and coagulation of the solid AgCl.
Co SACC Procedure page 5 12. While the solution is boiling obtain a clean, dry fritted glass crucible from the oven or your instructor. Use a pair of crucible tongs and a wire gauze held underneath the crucible for support as you transport it to your desk (this may have been demonstrated by your instructor). When cooled to room temperature mass the glass crucible on the analytical balance to ±0.0001 g. Record this in your notebook {mass crucible 2(heated)}. 13. After the color of the solution fades to orange remove the beaker from hot plate, use a magnetic wand and remove stirrer bar (or ask instructor to remove it) and begin cooling to ~10 C in an icewater bath (100 ml of ice/h 2 O in 400 ml beaker). Also cool about 15 ml of 0.01 M HNO 3 for washing. 14. Set up glass crucible, neoprene adapter, filter flask apparatus and connect to aspirator. Turn aspirator on full and check for vacuum leaks and for rate of flow. (If water does not come through fast enough, check with instructor, you may have an old glass crucible and needs to be replaced.) Filter solution through glass crucible first, and using your squeeze bottle squirt small portions of water behind precipitate and rinse all of it into the glass crucible. 15. Wash the precipitate with ~15 ml of cold 0.01 M HNO 3, in small portions of 4-5 ml, until the washings (1-3 drops) collected in a small test tube gives no turbidity when tested with a drop of 3M HCl. If the drops in your test tube are turbid (cloudy) continue washing with 0.01 M HNO 3 to free the precipitate collected from excess silver ions. The turbidity can be tested by breaking the vacuum above the neoprene adapter, removing the glass crucible and transferring any drops of the solution adhering near the bottom of the crucible to a small test tube, as demonstrated by your instructor. 16. When the precipitate has been collected wipe the outside of your glass crucible with moist paper towel and dry to remove oil and fingerprints. Place the glass crucible in a small, dry 50 ml beaker insert a piece of paper with your NAME, heated sample, and locker number between the crucible and the beaker and place it in the container provided for oven drying. This completes the chloride analysis for the heated sample.
Co SACC Procedure page 6 Day 3-Part 3. Spectrophotometric Analysis of Cobalt(II) ion in Co x (NH 3 ) y Cl z A colored substance absorbs visible light according to Beer-Lambert Law: A = log I I o = logt = εlc %T = log (I/I o ) x 100 This law describes the linear relationship between the absorbance (A) of a solution, and its concentration (c). ε=molar absorptivity (M -1 cm -1 ) l= length(cm) of the cuvette and is the distance light travels through the sample, c=concentration of the absorbing species (M). 1. Prepare six standard solutions of cobalt(ii) ion by diluting the 0.075 M stock solution with deionized water using a 10-mL graduated pipet. Make the dilutions according to the table below: Std# ml 0.075 M stock solution ml H 2 O 1 5.00 5.00 2 4.00 6.00 3 3.00 7.00 4 2.00 8.00 5 1.00 9.00 6 0.90 9.10 7 Blank Few ml water Cover standard solution with parafilm to prevent evaporation. Now prepare a sample of your compound containing cobalt below. 2. Mass out 1-1.2 g of the unknown purple compound to (±0.0001 g) on the analytical balance into a 50-mL beaker. Record this mass in your notebook. Cover the beaker with a watch glass, place it on a hot plate in the fume hood and heat until the solid sample liquefies, foams and turns blue. This process frees the cobalt ion from the other ligands in the complex. Remove the beaker from the hot plate and allow it to cool to room temperature (r.t.). Add 10 ml of deionized water and 1 ml of concentrated sulfuric acid to the sample. CAUTION: Concentrated H 2 SO 4 is a strong acid that will cause severe burns and may damage clothing, so handle with care! Dissolve any solid that remains by boiling the mixture gently on a hot plate. Cool the solution to r.t. Quantitatively transfer the contents of the beaker to a clean 100.0 ml volumetric flask. This is accomplished by rinsing the beaker (after careful transfer of the solution) with small portions (5 ml 2 to 3 times) of deionized water. All the washings are added to the volumetric flask. This will ensure that all of the solution goes into the flask. Swirl the solution in the volumetric flask and fill with deionized water from your wash bottle adding the last few drops with your plastic pipet to the fill mark. Stopper the flask and mix thoroughly (invert it several times). Transfer 5-6 ml into a small test tube and cover with parafilm. All seven samples plus the blank can now be read consecutively on the 200E spectrophotometer in the balance room. 3. First you will determine the wavelength of maximum absorption by scanning the visible spectrum with your first standard solution. You will use only two square cuvettes. One for the blank solution to calibrate the spectrophotometer and the other for all your absorbing samples. Set the scan mode on your spectrophotometer. A how to guide is in your Laboratory Module on the Canvas website! Rinse out the blank cuvette with small amounts of distilled water and fill it about 2/3 full. Insert the blank cuvette into the sample compartment close the lid and press auto zero on the membrane
Co SACC Procedure page 7 keypad. Once the Auto Zero measurement is complete, remove the cuvette with the blank solution. Set the measurement mode to A, minimum wavelength (low λ) to 360 nm and maximum wavelength (high λ) to 750 nm. Use the arrow key to select Next and (enter/select) to proceed to scan screen. Rinse your sample cuvette with the first standard solution and fill it about 2/3 full. Place the cuvette with your sample in the sample compartment (stage). Press to scan across selected wavelength range. Once the scan is complete use the λ knob or to move the green cursor line to identify the wavelength of maximum absorbance. Take picture of the scan and record λ max. 4. Record the %T (±0.1%T) of all your standard solutions and your unknown sample. (a) Press the home button to select the next mode. (b) Use the to select the Application mode: Live Display. (c) Use to select (text turns green to show which option is selected) Measurement Mode then use to toggle between %T and Abs. Set mode to %T. (d) Select Measurement λ and use or the λ knob to set the desired measurement wavelength. This may already be at the maximum wavelength desired. (e) Use arrow keys to select GO. Press to start the Live Display Mode. The displayed %T/Abs value updates every 2 to 5 seconds. To freeze the display, press. (f) Place a cuvette containing a blank solution in the sample compartment, press Auto zero. The screen displays the message Performing Auto Zero. When finished the display should show 100.0 %T. (g) Remove the cuvette with the blank solution and place the cuvette with your first standard in the sample compartment. If the display is frozen, press read. Record each %T value displayed. Rinse cuvette with the next standard and read %T. Read all subsequent standards and unknown Co-sample in the same cuvette. (h) Check your data with your instructor before you discard the standard and unknown samples.
Co SACC Procedure page 8 Day 4-Part 4. Thermal decomposition and volumetric analysis of NH3 in Cox(NH3)yClz You may determine the number of moles of ammonia per mole of your purple Co-complex by decomposing a known mass of your cobalt complex by heating in sodium hydroxide. This liberates NH3 gas, which you can trap as ammonium ion (NH4+) in a known quantity of standardized HCl. You can determine how much HCl reacted with the ammonia, by back-titration with standard NaOH, and thereby you can determine how many moles of NH3 were liberated from the complex. Co(NH3)nClm + NaOH Co2O3(s, black) + nnh3(g ) Xs Std HCl NH4+(aq) + left over HCl Reacted with Std NaOH to a bromocresol green endpoint ~H2O Procedure: 1. Set up the gas delivery apparatus as shown in the photo. The orange plastic clamp goes with your 250 ml Erlenmyer Flask (EF) reaction vessel, and the green plastic clamp goes with the 25 x 250 mm test tube used for the thermal decomposition of your synthesized Co-complex. Also set up two burets, one for your standardized NaOH and the other for your standardized HCl. The HCl buret can be shared by the entire bench to deliver the required amount of this reagent. Obtain approximately 60 ml of standardized NaOH and ~100 ml (more as needed) of standardized HCl in labeled beakers. Record their exact concentrations in your notebook. Rinse your burets with the appropriate solutions, fill and remove trapped air from the stopcock/buret tip area. Make sure that you fill the buret with the standardized NaOH and not with the ~3M NaOH! Use buret caps for the NaOH buret to prevent CO2 diffusion that will change the concentration. 2. Carefully deliver 30.00 ml of the standardized HCl into the clean 250 ml Erlenmyer Flask (EF) of the gas delivery apparatus (figure) from your HCl buret. Place EF on a magnetic stirrer plate. Add a magnetic stir bar (may be obtained from the instructor. Add ~20 ml of deionized water and 23 drops of bromcresol green indicator to the HCl
Co SACC Procedure page 9 solution in the EF. Record the color. Replace the glass tubing/rubber stopper assembly securely (tightly) into the neck of the EF. Wrap a rubber band around the plastic cap to secure the rubber stopper. 3. Mass out ~0.25 g of your purple complex on the analytical balance (record this mass to the nearest ± 0.0001 g) and put it in the large 25x250 mm test tube of the apparatus. Clamp the test tube about 16 inches above the bench top at a 45 angle as shown in the photo. Now add ~3 ml of 3 M NaOH to the complex in the test tube, and put in the stopper attached to the rubber tubing of the gas delivery apparatus. Wrap a rubber band around the plastic Keck clamp to secure the rubber stopper. Your apparatus is now charged and sealed. Turn the magnetic stirrer on and stir solution constantly while decomposing your Co-complex and collecting ammonia gas. 4. Ignite your Bunsen Burner and adjust the flame to obtain a blue inner cone. GENTLY (yes, seriously) start heating the test tube by moving the Bunsen Burner back and forth from bottom toward the top of the test tube but don t go beyond the upper end of the test tube because you will burn the rubber stopper. Move the Bunsen Burner flame back and forth several times along the length of the test tube to remove condensed water vapor that has collected in the upper portion of the test tube. You will have to apply this technique periodically to drive the NH 3 gas and water vapor into the gas collection flask (EF). As you proceed with the decomposition of the purple complex your goal is to get the solution to come to a gentle rolling boil, drive the ammonia gas and water vapor into the EF and trap it in the acidic solution contained within the EF. The rolling boil we are looking for is when the heated liquid is producing gentle bubbles consistently. The rolling boil can be achieved by passing the Bunsen Burner flame intermittently (back and forth) across the lower end of the test tube. When the solution comes to a gentle boil you can place the Bunsen burner on the desktop under the bottom of the test tube and adjust the flame to maintain this gentle rolling boil. Allow the magnetic stirrer to continually stir the solution in the EF. With a rolling boil the hot NaOH solution will decompose the complex, liberating NH 3 (g). As the magnetic stirrer mixes the solution in the EF the HCl solution will trap the ammonia gas given off as ammonium ions, NH 4 +. {Also check the rubber stopper on the EF periodically so it remains tight as you are collecting the ammonia gas.} Following the above directions the gas collection should take approximately 15-20 minutes. The decomposed complex will turn black (Co 2 O 3 ). Continue stirring the EF and gently heat the test tube close to dryness. Record in your notebook color changes of the complex, and of the bromocresol green indicator, if any. When the complex is decomposed completely, wait a few minutes, un-stopper the test tube and with a wash bottle rinse the glass delivery tube into the EF with small amount (~ 5 ml portion=aliquot) of distilled water. Set the Erlenmeyer flask aside to titrate (next paragraph). Clean the used test tube with a test tube brush and soap. Rinse with tap water followed by deionized water and set up for a duplicate ammonia determination. CAUTION: Once you have established a rolling gentle boil, stir the EF periodically to eliminate the chance of excessive gas pressure and loss of ammonia! 4. Titration: Record the initial volume in the NaOH buret, titrate the excess HCl in your 250 ml EF until you reach the bromcresol green end point, which is Blue (yellow-green-blue). Record the
Co SACC Procedure page 10 final volume in the buret and calculate the volume of NaOH used. Repeat analysis for a second determination.
Co Synthesis and Analysis of a Cobalt Coordination Complex Experimental Report Name: Partner s Name: Lab Section: MW/TTH/M-TH/F (circle) Experimental Data Part 1. Synthesis of Co x (NH 3 ) y Cl z Mass of CoCl 2 6H 2 O g Mass of purple product (obtained) g {Samples of your product will be used in subsequent analyses!} Part 2. Gravimetric determination of Chloride Ion in Co x (NH 3 ) y Cl z UNHEATED Mass of sample I (unheated) g Mass of sample II (unheated) g Mass of glass crucible (I) g Mass of glass crucible (II) g Mass(crucible + AgCl) (I) g Mass(crucible + AgCl) (II) g HEATED Mass of sample I (heated) g Mass of sample II (heated) g Mass of glass crucible (I) g Mass of glass crucible (II) Mass(glass crucible + AgCl) (I) g Mass(glass crucible + AgCl) (II) g Part 3. Spectrophotometric determination of Cobalt(II) ion in Co x (NH 3 ) y Cl z Mass of sample I (your product) g Mass of sample II (your product) g Standard Solution 1: ml of stock solution ml of water %T Standard Solution 2: ml of stock solution ml of water %T Standard Solution 3: ml of stock solution ml of water %T Standard Solution 4: ml of stock solution ml of water %T Standard Solution 5: ml of stock solution ml of water %T Standard Solution 6: ml of stock solution ml of water %T Blank (for calib.) 7: 0 ml of stock solution ~3 ml of water %T 100_(auto set) Wavelength of maximum absorbance = nm {Attach Photo of spectrophotometric scan of Absorbance v. wavelength} %T sample I (your product) %T sample II (your product)
SACC Report page 12 Experimental Data cont. Part 4. Thermal Decomposition and Volumetric Analysis of NH 3 in Co x (NH 3 ) y Cl z Mass of sample I (your product) g Mass of sample II (your product) g Molarity of standardized HCl M Molarity of standardized NaOH M 1 st Trial 2 nd Trial Initial reading HCl buret ml ml Final reading HCl buret ml ml Initial reading NaOH buret ml ml Final reading NaOH buret ml ml
SACC Report page 13 Calculations from Experimental Data: 5. Percentage Chloride Ion in Co x (NH 3 ) y Cl z Unheated samples Mass of sample I (data from P2.) g Mass of sample II (data from P2.) g Molar Mass of AgCl g/mol Molar Mass Cl g/mol Mass of glass crucible (I) g Mass of glass crucible (II) g Mass(glass crucible + AgCl) (I) g Mass(glass crucible + AgCl) (II) g Mass of AgCl (sample I) g Mass of AgCl (sample II) g Mass of Cl - (sample I) g Mass of Cl - (sample II) g Percent Cl - (in sample I) % Percent Cl - (in sample II) % Average %Cl(unheated) = % Heated samples Mass of sample I (data from P2.) g Mass of sample II (data from P2.) g Mass of glass crucible (I) g Mass of glass crucible (II) g Mass(glass crucible + AgCl) (I) g Mass(glass crucible + AgCl) (II) g Mass of AgCl (sample I) g Mass of AgCl (sample II) g Mass of Cl - (sample I) g Mass of Cl - (sample II) g Percent Cl - (in sample I) % Percent Cl - (in sample II) % Average %Cl(heated) = % What is the ratio (expressed as integers) of the average %Cl(heated) to the average %Cl(unheated)? {Show calculation below} From the integer ratio obtained above, How many chloride ions are present in your Co compound? ; How many are coordinated to Co? Briefly Explain how you determined this. Show calculations for each unique determination below and enter results in space provided above! (1) Mass AgCl (sample )= mass(glass crucible + AgCl) - mass(glass crucible) = (2) Mass of Cl - (sample )= (3) %Cl - (sample )= (4) Ratio of average {%Cl(heated)/ %Cl(unheated)}=
SACC Report page 14 6. Percentage Cobalt(II) ion in Co x (NH 3 ) y Cl z {Two determinations!} Mass of sample I (data from P3) g Mass of sample II (data from P3) g The equation of the Least Squares straight line (Excel s Trendline) from standard solutions: {Also attach your Standard Absorbance vs. Concentration output from Excel to your report!} Molar Mass Co = g/mol [Co] (sample I) = M [Co] (sample II) = M Mass Co (I) = g Mass Co (II) = g % Co (sample I) = % % Co (sample I) = % Average % Co = % Show calculations for one determination below! 7. Percentage of NH 3 in Co x (NH 3 ) y Cl z {Two determinations!} Mass of sample I (from data P4) g Mass of sample II (from data P4) g Molarity of standardized HCl M Molarity of standardized NaOH M Volume of HCl used (I) ml Volume of HCl used (II) ml Moles HCl used (I) mol Moles HCl used (II) mol Volume of NaOH used (I) ml Volume of NaOH used (II) ml Moles NaOH used (I) mol Moles NaOH used (II) mol mol NH 3 (I)=mole HCl mole NaOH = mol mol NH 3 (II)=mole HCl mole NaOH = mol Molar Mass of NH 3 g/mol Mass of NH 3 (I) g Mass of NH 3 (II) g %NH 3 (I) % %NH 3 (II) % Average %NH 3 = % Show calculations for one determination (following page)!
SACC Report page 15 (a) Calculate the number of moles of standardized HCl (use exact concentration) that you STARTED with in your 250 ml Erlenmyer Flask (EF). (b) Calculate the number of moles of standardized NaOH (use exact concentration) you added to neutralize the left-over HCl in your beaker. (c) Write the balanced equation for the reaction between HCl (aq) and NH 3(g). This reaction will use up some of your HCl. (d) Write the balanced equation for the reaction between HCl (aq) and NaOH (aq). This is how you will determine how much HCl was left over in each determination. (e) Calculate how many moles of HCl reacted with the ammonia trapped during each thermal decomposition (HCl start HCl left over ) (f) Calculate how many moles of ammonia were obtained from your complex. {See (c) above} (g) Determine the molecular mass of ammonia. (h) Calculate {use results from step (f)} the number of grams of ammonia liberated from your complex. (i) Using the mass of the sample of the complex you started with (from data P4 for each determination), calculate the % ammonia in the complex (g NH 3 /g complex) x 100.) (j) {Enter the results of all your calculations on the previous page}.
SACC Report page 16 8. Determination of the formula of Co x (NH 3 ) y Cl z Percentages % Co = %NH 3 = %Cl= Whole number ratios x = y = z = Formula of Compound: Molar mass of compound g/mol Show both Empirical formula calculations below and enter your results in the space provided above! 9. Percent yield of Co x (NH 3 ) y Cl z Mass of CoCl 2 6H 2 O used = g Actual yield of purple product = g Show calculations of theoretical yield, and %Yield {below} Theoretical yield g/mol Percent yield % 10. Questions/Problems (a) Estimate the maximum wavelength of visible light absorbed by the purple Co-complex ion that you synthesized or used for analysis. nm; (b) What is the oxidation state of Co in the complex ion? (c) Determine the crystal field splitting energy, o, in units of J/photon and kj/mol-photons. (d) Draw a Crystal Field Splitting diagram for your cationic complex ion. Speculate on whether the complex ion is high spin, low spin, paramagnetic, or diamagnetic. Fill the crystal field diagram with electrons on the metal and justify your choice.
Advanced Study Assignments SACC Report page 17 Name: Lab MW/TTH/M-TH/F (circle) Day 1-Part 1. Synthesis {To be turned in at the beginning of the laboratory period} 1. Materials Safety Data Sheets (MSDS) or (SDS) Do a short internet search on the MSDS for ammonia, hydrochloric acid, and hydrogen peroxide. Examine "Section 4 - First Aid Measures" of the MSDS. Determine what to do if concentrated ammonia, hydrochloric acid and hydrogen peroxide comes into contact with your skin or eyes. (a) Ammonia (NH 3 ): (b) HCl: (c) H 2 O 2 : 2. (a) Identify the reactant on which the percent yield will be based. (b) Why is H 2 O 2 used in the synthetic step of this experiment? (c) What is the origin of the white cloud that forms during the synthesis process? Write a balanced equation indicating its formation. (d) Calculate the molar mass (to 2 decimal places) of the limiting reactant, CoCl 2 6H 2 O, used in the total synthesis of the cobalt complex, the product. {The remainder of this experiment will we will elucidate the composition and structure of your synthetic endeavor.}
SACC Report page 18 Advanced Study Assignments Name: Lab MW/TTH/M-TH/F (circle) Day 2-Part 2. Chloride Analysis {To be turned in at the beginning of the laboratory period} 1. Why did we wait until part 2 of this experiment to mass the synthesized product made in part 1? 2. The balance used to mass out the samples to be analyzed is a(an) with a precision of ± g. 3. The sample to be analyzed for chloride is dissolved in deionized water. What chemical reagent, volume, and concentration must be added before precipitation of chloride is initiated?. 4. The reagent used to precipitate the chloride present in your synthesized product for both heated and unheated samples along with its concentration is, M. 5. Why do we carry out a turbidity test before oven drying our chloride containing product? 6. A student, Legna, massed out 10.12 g of chromium(iii)chloride hexahydrate to synthesize a transition metal coordination complex similar to what you are carrying out in your laboratory synthetic effort. She obtained 7.20 g of a complex containing Chromium, Cr, ammonia, NH 3, and chlorine, Cl. Samples of this compound were used to perform all analyses for the remainder of the advanced study assignments. The following data were obtained. {To the student: Please note that the synthesis and analysis of the compound in this and the following advanced study assignments are similar to but not identical to your experimental syntheses and analysis project!} Analysis 1. No heat applied Analysis 2. Heat applied Mass of compound used 0.4988 g Mass of compound used 0.4632 g Mass of crucible plus AgCl 19.2859 g Mass of crucible plus AgCl 21.0926 g Mass of crucible 18.4628 g Mass of crucible 20.3286 g M mass AgCl = 143.32 g/mol M mass Cl - = 35.45 g/mol (a) Determine the %Cl in each sample of the compound. Show calculations below! Not heated %, Heated % (b) Calculate the ration of %Cl(heated analysis)/%cl(unheated analysis). (c) Which one of the Cl-analyses gives the total chloride in the complex (heated or unheated analysis)? Briefly explain your reasoning. (d) What does the ratio of %Cl(heated)/%Cl(unheated) reveal about the number and location of the chlorides in this particular compound?
SACC Report page 19 Advanced Study Assignments Name: Lab MW/TTH/M-TH/F (circle) Day 3-Part 3. Spectrophotometric determination of %Cr in Cr x (NH 3 ) y Cl z {To be turned in at the beginning of the laboratory period} 1. In the Beer-Lambert Law, A=εlc, what does l stand for? 2. The data below were recorded every 25 nm in the visible spectrum from 450 nm to 750 nm for a solution that is 0.50 M in Cr 3+ ions using a spectrophotometer. {The spectrophotometer that we will use for the determination of the maximum wavelength of Absorption of a Co 2+ solution will scan the visible spectrum for you. You will take a photo of the result and include it in your report.} %Transmittance Wavelength (nm) Absorbance 65.1 700 52.2 675 24 650 8.2 625 4.3 600 3.1 575 7.8 550 16.3 525 32.1 500 35.3 475 37.2 450 (a) In the table above convert % Transmittance (%T) to Absorbance (A) using the relationship A = 2 log(%t). Use Excel, or similar program, plot absorbance versus wavelength and determine the approximate wavelength of maximum absorbance of Cr 3+ ion. Wavelength of ~maximum absorbance λ max = nm. Attach your plot to your report. (b) To determine the %Cr in the chromium-complex Legna synthesized, she massed out a 1.0240 g sample of the Crcomplex. She dissolved the sample in a volumetric flask by the procedure you use in part 3 of the experiment and made 100.0 ml of solution. From the standard curve of Absorbance versus concentration Legna obtain a [Cr 3+ ] = 0.03920 M. Calculate the %Cr in the complex.
SACC Report page 20 Advanced Study Assignments Name: Lab MW/TTH/M-TH/F (circle) Day 4-Part 4. Volumetric determination of %NH 3 in Cr x (NH 3 ) y Cl z {To be turned in at the beginning of the laboratory period} 1. What is a rolling boil? 2. In the titration of the solution in the EF, what indicator do we use and what is its color at the end-point of the titration? Indicator: Color at end-point: 3. Legnas chromium-complex analysis continued. (a) A 0.2536 g sample of the Cr-complex was thermally decomposed and the NH 3 released was collected in a 250 ml Erlenmyer flask containing 30.00 ml of 0.3065 M HCl solution and about 20 ml of deionized water. The solution was then titrated with standardized NaOH solution having a concentration of 0.2816 M. It required 11.92 ml of NaOH to reach the bromcresol green end point. Determine the %NH 3 in the sample. {Use procedure described in section 7. of experiment.} (b) Determination of the formula of Cr x (NH 3 ) y Cl z (i) from advanced study assignments 2, 3, 4(a) use the percentages obtained for Cl, Cr, and NH 3 and determine the formula of the Cr-complex Legna synthesized. (ii) Calculate the percent yield of Legna s synthetic endeavor.