Spring 2009 CH302 Worksheet 5 Answer Key—How to Systematically Work Harder and Harder Acid Base Calculations Exactly the Same Way: Proof that the Seven Steps to Solving Acid Base Problems Work 1 Remove the spectator ions 2 Are there any strong acids or bases 3 Are there any weak acids or bases 4 Do I neutralize (are there both acids and bases and is at least one of them strong?) 5 Neutralize: convert everything to moles, write down neutralization reaction, perform limiting reagent calculation, convert back to molarity if necessary) 6 Select the appropriate acid base calculation and solve 7 Convert to appropriate final form (pH, pOH, H+, OH-) using 14 = pH + pOH and 14 = pKa + pKb Important: These calculations are based upon the following important assumptions: • Strong acids and bases completely dissociate • Weak acids and bases do not dissociate significantly (typically they will have K values <10-3) • The dissociation of water does not contribute significantly to pH (concentrations of dissolved solutions are large, > 10-4, and the K values are not near Kw, >10-11) In a nutshell, all of these problems are worked at high concentrations for a single equilibrium. When we get to complex equilibria you will learn how to tackle problems for which the assumptions do not hold. 1. What is the pOH of a 0.1 M HClO4 solution? What kind of acid base problem was this? Strong acid with a pOH of 13. 2. What is the pH of a 0.1 M RbOH solution? What kind of acid base problem was this? Strong base with a pH of 13 3. What is the [H+] of a 0.1 M malonic acid with a Ka of 10-9 solution? What kind of acid base problem was this? Weak acid with a pH of 5 4. What is the pH of a 0.1 M lithium malonate solution? (Need a Kb? Look at the problem above.) What kind of acid base problem was this? Weak base with a pH of 11 5. What is the [OH-] of a 0.01 M methylamine solution of Kb = 10-6? What kind of acid base problem was this? Weak base with a [OH-] of 10-4 M 6. What is the pOH of a 0.01 M CH3NH2Br solution? (Need a Ka? Look at the problem above.) What kind of acid base problem was this? Weak acid with a pOH of 9 7. What is the pH when equal volume mixtures of 0.2 M HClO4 and 0.2M LiClO4 are mixed? What kind of acid base problem was this? Strong acid with a pH of 1 (remember the volume change)8. What is the pH when 100 ml of 0.1 M HClO4 and50 ml of 0.25 M Ba(OH)2 are mixed? What kind of acid base problem was this? Neutralization. Final solution .015moles OH-/.15 L = .1 M pH = 13 9. What is the pH when 1 liter of 0.1 M HClO4 and 1 liter of 0.5M Ba(OH)2 are mixed? (this is the first problem for which you need a calculator) What kind of acid base problem was this? Strong base with a pH of 13.65 10. What is the pOH when 100 ml of 0.1 M malonic acid and 100 ml of 0.1 M sodium malonate are mixed? What kind of acid base problem was this? Buffer with a pOH of 5 10. What is the pH when 100 ml of 0.1 M methylamine and 100 ml of 0.1 M CH3NH2Br are mixed? What kind of acid base problem was this? Buffer with a pH of 8 The next four calculations represent the titration of a weak base with a strong acid. Note the pH gets smaller and smaller as more acid is added. 11. What is the pH when no HBr is added to 100 ml of 0.1 M sodium malonate? What kind of acid base problem was this? Weak base with a pH of 11 12. What is the pH when 50 ml of 0.1 M HBr is added to 100 ml of 0.1 M sodium malonate? What kind of acid base problem was this? Buffer with a pH of 9 13. What is the pH when 100 ml of 0.1 M HBr is added to 100 ml of 0.1 M sodium malonate? What kind of acid base problem was this? It is a weak acids with pH of 5.15 14. What is the pH when 110 ml of 0.1 M HBr is added to 100 ml of 0.1 M sodium malonate? What kind of acid base problem was this? Excess strong acid with pH of 2.3 The next four calculations represent the titration of a weak acid with a strong base Note the pH gets larger and larger as more base is added. 15. What is the pH when no LiOH is added to 200 ml of 0.05 M CH3NH2Br? What kind of acid base problem was this? Weak acid with pH of 4.65 16. What is the pH when 100 ml of 0.05 M LiOH is added to 200 ml of 0.05 M CH3NH2Br? What kind of acid base problem was this? Buffer with a pH of 817. What is the pH when 200 ml of 0.05 M LiOH is added to 200 ml of 0.05 M CH3NH2Br? What kind of acid base problem was this? Weak base with pH of 10.2 18. What is the pH when 250 ml of 0.05 M LiOH is added to 200 ml of 0.05 M CH3NH2Br? What kind of acid base problem was this? Excess strong base with pH of 11.7 19. What is the pH when 10 ml of 0.1 M HClO3 is added to 100 ml of 0.1 M methylamine and 100 ml of 0.1 M CH3NH2Br? What kind of acid base problem was this? Buffer with pH of 7.95 (note it is a little less than pH 8 because we added a little bit of strong acid to a 1:1 buffer with pKa = 8. 20. What is the pOH when 20 ml of 0.001 M KOH is added to 200 ml of 0.01 M malonic acid and 200 ml of 0.02 M sodium malonate are mixed? What kind of acid base problem was this? Buffer with a pOH of 4.69 Super-duper do it in your head pH problem. What is the pH when 10 ml of 0.1 M HClO3 and 20 ml of 0.05M Ba(OH)2 are added to 150 ml of 0.1 M methylamine and 75 ml of 0.2 M CH3NH2Br? Hint, put away your calculator and do it in your head. What kind of acid base problem was this? This is a buffer with pH 8. Note that the strong acid and strong base are present in equal amounts and neutralize each other. Also note this is a 1:1 buffer with pH =
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