Smith-Palmer Chemistry 265 Analytical and Environmental 2010 Part A Short Snappers [2] 1. Calculate the ph of a solution prepared by mixing 2.0 ml of a strong acid solution of ph 3.00 with 3.0 ml of a strong base solution of ph 10.00. H + : 2.0 ml x 10-3 M = 2.0 x 10-3 mmol H + OH - : 3.0 ml x 10-4 M = 3.0 x 10-4 mmol OH - Difference = 1.7 x 10-3 mmol H + / 5.0 ml = 3,4 x 10-4 M ph = 3.47 [1] 2. State Beer s law. A = εbc [1] 3. What is the general principle behind all chromatography (2 words)? [2] 4. What is the full name for EDTA? Differential holdup Ethylenediaminetetraacetic acid How many metal ions can each EDTA ion bind with? one [3] 5. Write the charge balance equation for an aqueous solution of Al 2 (SO 4 ) 3. 3[Al] + [H + ] = 2[SO 4 2- ] + [H SO 4 - ] + [OH - ] [4] 6. The ph of blood is 7.40. What is the ratio of bicarbonate to carbonic acid in the blood. For carbonic acid, pk a1 = 6.351 and pk a2 = 10.329 7.4 = 6.35 + log ([HCO 3 - ]/[H 2 CO 3 ]) [HCO 3 - ]/[H 2 CO 3 ] = 11 Is this a good buffer system? No ratio is larger than 10
What will be the ph if the blood is diluted by a factor of 5? 7.4 [1] 7. Define the cathode in a galvanic cell. Where reduction occurs [5] 8. Calculate the theoretical potential for the following cell. Indicate whether it is galvanic or electrolytic. Pt, H 2 (0.740 bar) H + (5.67 x 10-7 ) Pd 2+ (0.0120 M) Pd Pd 2+ + 2e - Pd E o = 0.915 V E c =0.915-0.0559 2 log 1 0. 0120 = 0.858 V E a 0-0.0592 2 0.740 log 7 2 (5.67x10 ) = -0.366 V E cell = 0.858 V (-0.366 V) = 1.22 V galvanic [2] 9. What are some properties that are necessary for any reaction that is going to be used as the basis for a titration. Fast Quantitative Available indicator Known stoiciometry
[3] 10. Name a reference electrode that is commonly used in the lab. Write the Nernst equation for the half-reaction involved in the electrode and explain why the potential stays constant. Standard calomel electrode Hg 2 Cl 2 + 2e - 2 Hg (s) + 2 Cl - E = E o 0.0592 - log [Cl - ] 2 2 Use a saturated solution of chloride so the concentration will always be constant = nothing else affects the equation Also could use silver/silver chloride electrode NOT the hydrogen electrode [2] 11. One of your friends said they were going to titrate Na Cl with silver nitrate and determine the endpoint potentiometrically. Your other friend did not understand (they have been skipping class) and asked you to explain what that meant. Please write out how you would explain this. Use silver wire and reference electrode in solution. Measure potential as silver nitrate is added Jump occurs at endpoint [3] 12. Later you talked again with the friends who were going to titrate Na Cl with silver nitrate and they said they were just going to use an indicator. Suggest an indicator they could use for that particular titration. Explain why it changes colour at the endpoint. What is the colour change which occurs. Chromate it will ppt with silver just past equivalence point will see red ppt Dichlorofluorescein will adsorb on AgCl ppt once there is an excess of silver ions making it positively charged Will create a pink ppt
[2] 13. Write the reactions whereby carbon dioxide dissolves in water to form carbonic acid, then show its two step dissociation. CO 2 + H 2 O H 2 CO 3 H 2 CO 3 H + + HCO 3 - HCO 3 - H + + CO 3 2- [4] Answer Four of the following: What percentage of the air is carbon dioxide? 0.04% Would carbon dioxide dissolve more in base or acid? base What happens to the amount of dissolved carbon dioxide when water is heated? decreases What is happening when we say marble buildings dissolve as a result of acid rain. Write the equation. 2 H + + CaCO 3 2 HCO 3 - + Ca 2+ How will the ph of seawater change (lower or higher) as CO 2 levels in the atmosphere increase? lower [2] 14. Draw a block diagram of a UV-visible absorption spectrometer. Label the boxes. source monochromator sample cell detector
[4] 15. What are the two oxidation states for iron? 2 +, 3 + What one is most stable in an aqueous acidic solution? 3 + What chemical can cause ferric ions to be reduced to ferrous ions? Sn 2+ Give the balanced equation for the reaction. 2 Fe3+ + Sn2+ 2Fe2+ + Sn4+ [1] 16. What is the voltage for a cell where the reaction has reached equilibrium? 0
Part B: Contaminated Site You are an environmental chemist and have been called to examine a site where a new industry wants to build a plant. They are concerned about the contamination that may have been left in the soil by previous owners, and also wonder whether the water in the stream will be suitable for them to use in any of their applications. [6] 1. Choose three of the following water tests. Explain what it tests for and what is done to carry out the test. Alkalinity Hardness Biological oxygen demand Acidity Dissolved salts
2. There was a big pile of soil/dirt at one corner of the site. There seemed to be a mix of pale and dark particles. A sample weighing 3.50 g was collected. After examining the sample in the microscope, it was determined that all the particles were roughly the same size and almost a mm in diameter. About a third of the particles were light coloured. These particle are thought to contain silica. The dark particles will be analysed for their chromium content by a method that gives a standard deviation of 4%. [2] If about 10 4 particles are collected in the sample, what will be the % error in the sampling of the dark particles? [1] What will be the total combined percentage error for the sampling and analysis? s T 2 = (4 %) 2 + (0.7 %) 2 s T 2 4% [1] If the white particles were silica particles, what could be used to remove or dissolve them? Hot HF [3] 3. What is the basis for the hold-up in ion-exchange chromatography? Explain how you could separate a neutral species from a charged species using ion exchange chromatography. Differential hold-up because of attraction of ions to oppositely charged functional groups on the stationary resin. [1] 4. One of the companies on the site previously had used a lot of lead in their factory. You are trying to decide where it might have ended up on the site. Why would the presence of lead be a problem to the environment? Toxic to living things
[2] The K sp for PbCl 2 is 1.70 x 10-5. What is the solubility of PbCl 2 in water? Let solubility = x K sp = [Pb 2+ ][Cl ] 2 = x (2x) 2 1.70 10-5 = 4x 3 Solubility = x = 0.0162 M [4] What is the solubility of PbCl 2 in 0.040 M NiCl 2? 1.70 10-5 S (s + 0.08) s(s + 0.08) 2 assume 0.08 > s s(0.08) 2 = s(0.077 + 0.08) 2 0.00265 M Note: Necessary to solve cubic- but not expected here [2] If the PbCl 2 was being dissolved in a solution containing 0.8 M Na 2 SO 4, would it be more or less soluble than when it was put in water? (No numerical answer required) Why? PbSO4 is insoluble Less soluble
[5] 5. In one area of the site there is a container labeled chloroaniline. The purity of the contents needs to be determined. Chloroaniline reacts with picric acid to form amine picrates which absorb strongly at 359 nm (absorptivity = a = 125 L. g -1.cm -1 ). A 0.0265 g sample from the container is reacted with picric acid and diluted to 1.00 L. The absorbance of the solution was measured in a cuvet (test-tube) with a diameter (pathlength) of 1.00 cm. The absorbance was 0.547. What is the percentage of choroaniline in the substance in the container? A = a b c 0.547 = (125 Lg -1 cm -1 ) (1 cm) c 0.00432 g/l = c Assume 1 L sample so total 0.00433 g % = 100% = 16.3 %
Part C Titrations [4] 1. In general, at the endpoint of a titration, one wants a steep jump. Why? Defines end point better Sudden change of indicator There are a number of variables that influence how big and/or steep the jump at the endpoint is. Tell me about them. You can write in note form. Concentration of analyte Concentration of titrant Speed of reaction K a or K f or K sp
[3] 2. A 10 ml aliquot of a solution of sodium glycinate, the sodium salt of the amino acid glycine, was titrated with 0.0550 M hydrochloric acid. The reaction was followed using a ph electrode, and the titration was continued until the second endpoint had been passed. The jump for the second endpoint occurred when 24.70 mls of the acid had been added. For glycine, pk a1 = 2.35 pk a2 = 9.778 Sketch the titration curve. Mark in phs at important points in the titration [3] What was the concentration of the sodium glycinate? Can use the 1 st endpoint Then 1 : 1 0.0550 mmol/ml 12.35 ml = 0.67925 mmol [sodium gylc] = 0.0679 M [2] What is the ph at the first equivalence point? = 6.07
[4] What is the ph at the second equivalence point? [2] What is the concentration of glycinate (L - ) in the solution at the equivalence point? [L - ] = K a2 [2] It was hard to decide exactly where the endpoint was. Describe a way to massage the data to allow the endpoint to be determined very exactly. Derivative [1] If we had stopped collecting data a little way before the endpoint, what could you do with your data to be able to find the endpoint? Gran plot
Part D Calculation Fun- Choose any two. [5] 1. Calculate the formation constant, Kf, for Ag(CN) 2 - if the following cell develops a potential of 0.625 V. Ag 2 - (7.50 x 10-3 M), CN - (0.0250 M) SCE The potential for the SCE = 0.244 V Ag + + 2CN - Ag(CN) 2 - Ag + + e - Ag (s) E o = 0.799 V 0.244 V- 0.625 = -.381 V -.381 V = K f = { Rearrange to get [Ag + ] and substitute in Nernst above K f = 1.4 x 10 19
[5] 2. Calculate pni 2+ for the solution that results when 50.0 ml of 0.0500 M EDTA is titrated into 50.00 ml of 0.0300 M Ni 2+ at ph = 3.0. K f for NiY 2- is 4.2 x 10 18 At ph 3.0, α 4 for EDTA =2.5 x 10-11 Also calculate pni 2+ at the equivalence point in the titration. XS EDTA = 1.0 mmol pni = 7.85
[5] 3. What is the ph of a solution of 0.010 M H 2 O 2? Ka = 2.2 x 10-12 Ka = [H + ] total = [H + ] water + [H + ] peroxide [H + ] total [OH - ] = Kw [H + ] water =[OH - ] [H + ] total = [OH - ] + [P - ] = + [H + ] total = 4.8 x 10-7 ph = 6.32
[5] 4. What is the ionic strength of a solution that is 0.033 M in Mg(IO 3 ) 2? Activity Coefficients( ) at Indicated Ionic Strengths Ionic 0.001 0.005 0.01 0.05 0.1 strength Ba 2+ 0.868 0.744 0.67 0.46 0.38 - IO 3 0.964 0.928 0.902 0.82 0.78 The solubility of Ba(IO 3 ) 2 in a solution that is 0.033 M in Mg(IO 3 ) 2 is 1.56 x 10-6 M. Using activities, calculate the Ksp for Ba(IO 3 ) 2.