THE MOLE CONCEPT III Applying Moles to Chemical Solutions ADEng. Programme Chemistry for Engineers Prepared by M. J. McNeil, MPhil.

Transcription

1 THE MOLE CONCEPT III Applying Moles to Chemical Solutions ADEng. Programme Chemistry for Engineers Prepared by M. J. McNeil, MPhil. Department of Pure and Applied Sciences Portmore Community College Main Campus

2 LECTURE OBJECTIVES PART THREE Define molarity in terms of its mathematical formula. Calculate moles, liters, or molarity of a given solution. Explain how to make a standard solution. Perform various (volumetric) titrimetric analyses: acid/base and redox types.

3 MOLES AND SOLUTIONS MOLARITY AND TITRATION

4 NATURE OF SOLUTIONS Dilute Small amount of solute for given solvent Concentrated Large amount of solute for given solvent Saturated Maximum amount of solute for given solvent. These terms are qualitative, not quantitative, and are open to interpretation.

5 WHAT IS CONCENTRATION? CONCENTRATE!!!!!! How many stuff you have in a certain amount of space? Stuff = solute Space = volume (solvent) What if you have lots/less of stuff or space? Stuff = A, I, M Concentration is just like sweetness of a solution. Imagine: A sugar solution contains 10.0g of sugar per dm 3 of solution and another contain 2.0 g sugar per dm 3 of solution. The more concentrated one will be sweeter. Chemists need to know the [solution]. N.B cm 3 (ml) = 1 dm 3 = 1 L

6 IMPORTANCE OF CONCENTRATION Water must be tested continually to ensure that the [contaminants] do not exceed established limits. These contaminants include trace metals, pesticides, bacteria, and even the byproducts of water treatment. Food additives must be in correct concentrations. A driver is legally impaired at 0.08 mg/ml blood alcohol content. Prescription drugs in the correct concentration make you better. We may express results in any form of the analyte. E.g.: Water Hardness due to calcium ion is expressed as ppm CaCO 3. [P & D I] Bleach in swimming pools. In an enzyme-catalyzed reaction, the more enzyme added to the experiment increases the reaction rate and viceversa. Chemists control the concentration of chemicals using the concepts we will develop in this unit.

7 CONCENTRATION OF SOLUTIONS Often scientists carry out experiments where the chemicals involved are dissolved in water. A scientist must be able to work out how much of a substance is dissolved in a certain volume of solution. Behavior of solutions depend on compound itself and on how much is present, i.e. on the concentration. Two solutions can contain the same compounds but behave quite different because the proportions of those compounds are different. Concentration of a solution: the more solute in a given volume of solvent, the more concentrated. 1 tsp salt (NaCl)/cup of water vs 3 Tbsp salt/cup water

8 EXPRESSING CONCENTRATION - MOLARITY Need to make solutions with precise concentrations. Most common unit of solution concentration is: Simplified: M=n/v is M=g/mm x L, g meaning grams, mm meaning the molar mass of the solution Molarity (M) or molar concentration, [ ], is the term used to express concentration. It is a measure of the number of moles of solute per litre of solution. Calculating molarity: Units of molarity: moles per litres mol/l or mol/dm 3 or mol dm -3. Be careful not to get molarity and moles mixed up. Moles measures the amount of material. Molarity measures the [concentration]. A liter of solution containing 1 mol of solute is a 1M solution, which is read as a one-molar solution. moles solute molarity = liters solution M moles Liter A liter of solution containing 0.1 mol moles of solute is a 0.1 M solution ml

9 UNITS OF VOLUME One Litre (1L) or 1000 cm 3 (ml) 1 ml or 1 cm 3 1 millitre (ml) = about 20 drops 1 L = qt 1 fluid once (fl oz) = ml A 1.00 molar (1.00 M) solution contains 1.00 mol solute in every 1 liter of solution.

10 MOLARITY - MOLES - VOLUME Molarity (M) = Molarity (M) = moles of solute volume of solution in liters mol Volume (L)

11 CALCULATING MOLARITY What is the molarity of a solution if it contains 2.0 moles of potassium nitrate in 4.0 L of solution? If you re given grams, you have to convert grams to moles. Intravenous(IV) saline solutions are often administered to patients in the hospital. One saline solution contain 0.90 g NaCl in exactly 100 cm3 of solution. What is the molarity of the solution. What is the molarity of a solution if it contains 20 grams of NaOH in 2.0 L of solution? Calculate the molarity of a solution prepared by dissolving 25.6 grams of Al(NO 3 ) 3 in 455 ml of solution. Now that you know how to calculate the molarity of a solution, how would you prepare one in the laboratory?

12 MOLARITY PRACTICE 1. What is the molar NaCl concentration if you have 0.5 mol NaCl in 1.00 L of solution? Come on and wrestle these questions to the ground. You can do it! 2. How many moles of AgNO 3 are present in 25 ml of a 0.75 M solution? g KNO 3 dissolves in a 233 ml solution. What is molarity? g KOH is dissolved in 250. ml of water, calculate the molarity.

13 INTERCONVERSION OF MOLARITY, VOLUME AND MOLES 1. Household laundry bleach is a dilute aqueous solution of sodium hypochlorite (NaClO). How many moles of solute are present in 1.5 L of 0.70 M NaOH? 1. Formalin, HCHO, is used in preserving specimens. How many grams of HCHO must be used to prepare 2.5 L of 12.3 M formalin? 2. How many moles are there in 205. ml of a M solution? 3. How many moles of HCl are present in 2.5 L of 0.10 M HCl? 4. How many grams of NaOH are required to prepare 400. ml of 3.0 M NaOH solution? 2. What volume of a 0.10 M NaOH solution is needed to provide 0.50 mol of NaOH? 3. How many grams of CuSO 4 are needed to prepare ml of 1.00 M CuSO 4?

14 MORE PRACTICE QUESTIONS 1. How many grams of nitric acid are present in 1.0 L of a 1.0 M HNO 3 solution? 2. Calculate the number of grams needed to produce 1.00 L of these solutions: a) 1.00 M KNO 3 b) 1.85 M H 2 SO 4 c) 0.67 M KClO 3 3. Calculate the # of grams needed to produce each: a) 0.20 L of 1.5 M KCl b) L of M HCl c) 0.20 L of 0.09 mol/l AgNO 3 d) 250 ml of 3.1 mol/l BaCl 2 4. Give the molarity of a solution containing 10 g of each solute in 2.5 L of solution: a)h 2 SO 4 b)ca(oh) Describe how 100 ml of a 0.10 mol/l NaOH solution would be made. 63 g 101 g 181 g 22 g 3 g 82 g 1.75 g For more lessons, visit kg mol/l mol/l

15 PREPARING MOLAR SOLUTIONS FROM PURE SOLIDS Standard solutions are reagents of known concentration (mol dm -3 ) If a solution contains 1 mole of a substance in 1 dm 3 of solution, it is usually written as 1 M (1 mol dm -3 ) So, 1 M NaCl means there is 1 mole of NaCl in 1 dm 3 of solution. Weigh out a solid solute and dissolve in a given quantity of solvent. Dilute a concentrated solution to give one that is less concentrated. How would you prepare 1 M NaCl solution? Mass conc ( conc. In gdm -3 ) :Mass (in grams) of a substance dissolved in 1dm -3 of solution. Mathematically, Mass(g) Vol(dm 3 )

16 PREPARING 1.0 MOLAR SOLUTION One liter of a 1.00 M NaCl solution need 1.00 mol of NaCl weigh out 58.5 g NaCl (1.00 mole) and add water to make 1.00 liter (total volume) of solution.

17 STEPS TO PREPARE MOLAR SOLUTIONS FROM PURE SOLIDS STEP 1: Calculate the mass of the solute needed using the molarity definition and accounting for the desired concentration and volume. STEP 2: Weigh out the mass of the solute on an analytical balance. STEP 3: Transfer the solute in an appropriate volumetric flask. STEP 4: Fill flask about half full with distilled water and mix. STEP 5: Fill to calibration mark and invert to mix

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19 PREPARING 0.10 M NaOH 1. Calculate # of grams required to make 100 ml of a 0.10 M solution of NaOH. 2. Get volumetric flask, plastic bottle, 100 ml beaker, eyedropper. Rinse all with tap water. 3. Fill a beaker with distilled water. 4. Pour ml of H 2 O from beaker into flask. 6. Mix (by swirling) until the NaOH is dissolved. 7. Add distilled H 2 O to just below the colored line. 8. Add distilled H 2 O to the line using eyedropper. 9. Place solution in a bottle. Place label (tape) on bottle (name, date, chemical, molarity). Place bottle at front. Rinse & return equipment. 5. Weigh NaOH. Add it to flask. Do step 5 quickly.

20 To make a 0.5-molar (0.5M) solution, first add 0.5 mol of solute to a 1-L volumetric flask half filled with distilled water. Swirl the flask carefully to dissolve the solute. Fill the flask with water exactly to the 1-L mark.

21 PRACTICE MAKING MOLAR SOLUTIONS Describe how you would prepare ml of a M solution of CoCl 2. How many ml of a stock solution of 4 M KI would be needed to prepare 250 ml of 0.76 M KI? L x moles x g = 2.60 g 1 L 1 mole Weigh 2.60 g of CoCl 2 How would you prepare 0.1 M of sodium carbonate solution in the lab? Dissolve in water Transfer to a 100 ml volumetric flask and fill to the line 13. You need 250 ml of 0.2 M NaCl, but all you have is a 1 M solution. How do you prepare the required solution?

22 HOW WOULD YOU PREPARE MOLAR SOLUTIONS FROM LIQUIDS Solutions of exact concentrations of sulphuric acid for e.g. cannot be made up directly because it absorbs moisture from the atmosphere. Some distilled water can be placed into the desired volumetric flask and the relevant volume of the acid is run into from a burette. Solutions of approximate concentrations are made up and their exact concentrations are then found by titration. How would you prepare approximately 0.1 M H 2 SO 4 solution.

23 VOLUMETRIC PRINCIPLES - STANDARDS PRIMARY (1 o ) STANDARD A highly purified compound used as a reference material in titrimetry. Properties High purity Stable in air Independent of relative humidity Readily available Reasonable solubility Large formula weight React stoichiometrically fast E.g. Potassium Acid Phthalate, KHC 8 H 4 O 4 (FW ), Na 2 CO 3, K 2 Cr 2 O 7 SECONDARY (2 o ) STANDARDS This standard do not meet the properties as a primary standard, but are available in sufficient purity Desirable properties of: Prepared from primary standard Stable Reacts rapidily and completely with analyte Reacts selectively with analyte E.g. NaOH, KOH, Ba(OH) 2, HCl, HNO 3, HClO 4, KMnO 4, Na 2 S 2 O 3

24 PRINCIPLES OF VOLUMETRIC ANALYSIS DIRECT METHOD Dissolve carefully weighed quantity of primary (1 o ) standard; dilute to known volume. INDIRECT METHOD Titrate weighed quantity of primary (1 o ) standard. Titrate weighed quantity of secondary (2 o ) standard. Titrate measured volume of other standard solution..

25 TITRATION Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis. There are several types of titrimetric analysis. Acid-Base Titration HA + B BH + + A - Redox Titration, Red 1 + Ox 2 Ox 1 + Red 2 ph Titration Thermometric Titration Complexometric Titration Conductimetric Titration

26 VOLUMETRIC APPARATUS Conical flask Burette Pipette beaker Indicator for volumetric analysis

27 TITRIMETRY TERMS Titrimetry - determination of analyte by reaction with measured amount of standard reagent. Standard Solution (titrant) - reagent of known concentration. Titrand?? End Point - the occurrence of an observable physical change indicating that the equivalence point is reached. Might differ from eq. pt.! Titration - slow addition of titrant to analyte solution from a volumetric vessel (burette) Equivalence Point - reached when amount of added titrant is chemically equivalent to amount of analyte present in the sample.

28 IMPORTANCE OF TITRATION To determine the concentration of an analyte in a solution. To determine the proticity of an acid. To determine the stoichiometry of a chemical equation. To determine the percentage purity of a mixture. Stoichiometric Calculations: The Workhorse of the Analyst

29 TITRATION

30 MEASURING VOLUMETRIC DILUTION Many laboratory chemicals such as acids are purchased as concentrated solutions (stock solutions). e.g. 12 M HCl 12 M H 2 SO 4 More dilute solutions are prepared by taking a certain quantity of the stock solution and diluting it with water. In a dilution water is added. volume increases. concentration decreases. What about the number of moles?

31 MAKING DILUTIONS What effect does dilution have on the total moles of solute in a solution? Diluting a solution reduces the number of moles of solute per unit volume, but the total number of moles of solute in solution does not change. moles solute = moles solute before dilution after dilution

32 ACID-BASE TITRATIONS Solution Stoichiometry Remember: reactions occur on a mole to mole basis. For pure reactants, we measure reactants using mass 1. How many moles of HCl are present in 2.5 L of 0.10 M HCl? For reactants that are added to a reaction as aqueous solutions, we measure the reactants using volume of solution. 2. How many grams of CuSO 4 are needed to prepare ml of 1.00 M CuSO 4?

33 VOLUMETRIC TITRATION CALCULATIONS Solution stoichiometry practice 1. If 25.0 ml of 2.5 M NaOH are needed to neutralize (i.e. react completely with) a solution of H 3 PO 4, how many moles of H 3 PO 4 were present in the solution? 48.0 ml of Ca(OH) 2 solution was titrated with 19.2 ml of M HNO 3. Determine the molarity of the Ca(OH) 2 solution in mol dm -3 and g dm If ml of 2.5 M NaOH are needed to neutralize 50.0 ml of an H 3 PO 4 solution, what is the concentration (molarity) of the H 3 PO 4 solution?

34 LIMITING/EXCESS REACTANT Potassium superoxide, KO 2, is used in rebreathing gas masks to generate oxygen. 4KO 2 (s) + 2H 2 O(l) 4KOH(s) + 3O 2 (g) a. How many moles of O 2 can be produced from 0.15 mol KO 2 and 0.10 mol H 2 O? b. Determine the limiting reactant. 4KO 2 (s) + 2H 2 O(l) 4KOH(s) + 3O 2 (g) 0.15 mol 0.10 mol? moles Two starting amounts? Where do we start? Hide one 79.1 g of zinc react with 0.90 L of 2.5M HCl. Identify the limiting and excess reactants. How many liters of hydrogen are formed at STP? 34

35 REDOX TITRATION Read up!!!!

36 END OF SHOW With every mole calculations, You must never