All Notes, Books, Etc., Must Be Placed out of Sight. Please Read Each of the Problems Carefully s2

Chemistry 1212 1 Name______

Chemistry 1212

Exam III

Spring, 2002

Name______

All notes, books, etc., must be placed out of sight. Please read each of the problems carefully. If something is not clear, please ask.

The exam consists of 6 problems on 4 pages with a Take-Home extra credit on the fifth page. Make sure your exam is complete. The exam is worth 100 points with each of the problems labeled as to its point value.

Answers should be placed in the space provided and written legibly. If I cannot read it -- it is wrong. You MUST show your work clearly for each problem in order to receive full credit. Please include an explanation whenever asked to or whenever necessary to make your answer clear. If I cannot understand it -- it is wrong. Please put your name on every page of the exam.

There are scratch papers and periodic tables available for your use. If you require more data, please ask.

As usual, no cheating is allowed. Good Luck!

DATA

Ksp(PbI2) = 7.1 x 10-9 Kw = 1.0 x 10-14 (25 ºC)

Ksp(AgI) = 8.5 x 10-17

Kb(NH3) = 1.8 x 10-5 Ka(NH4+) = 5.6 x 10-10

Ka(HAc) = 1.8 x 10-5 Kb(Ac¯) = 5.6 x 10-10

Ka(HCN) = 6.2 x 10-10 Kb(CN¯) = 1.61 x 10-5

1.(15) What must be the molarity of an aqueous solution of trimethylamine, (CH3)3N, if it has a pH = 11.12? Show all of your work clearly for full credit.

(CH3)3N + H2O  (CH3)3NH+ + OH¯ Kb = 6.3 x 10-5

2.(15) Sorbic acid, HC6H7O2 (Ka = 1.7 x 10-5) is widely used in the food industry as a preservative. For example, its potassium salt (potassium sorbate) is added to cheese to inhibit the formation of mold. What is the pH of 0.37 M KC6H7O2 (aq)? Show all work clearly for full credit.

3.(20) You wish to prepare a buffer solution with pH = 9.45.

a. How many grams of (NH4)2SO4 (MW = 132) would you add to 425 ml of

0.258 M NH3 (MW = 17) to do this. Assume that the solution’s volume remains constant. Show all of your work clearly for full credit.

b. Which buffer component, and how much (in grams), would you add to 0.100 L of the buffer in part (a) to change its pH to 9.30? Assume the solution’s volume remains constant. Show all of your work clearly for full credit.

4.(10) Vinegar is approximately a 5.7% acetic acid solution. This information and the density (d = 1.0 g/ml) is enough for you to calculate (i.e. on the final exam) that this is a 0.95 M HAc solution. Determine the pH of a 0.750 L sample of this vinegar. Show all work clearly for full credit.

5.(20) What is the pH at the equivalence point for a titration of 100 ml of 0.250 M HCN with 0.100 M NaOH? Show work clearly for full credit.

6.(20) KI (aq) is slowly added to a solution with [Pb+2] = [Ag+] = 0.10 M.

a. Which precipitate should form first, PbI2 or AgI? Show all work clearly for full credit.

b. What concentration of the first cation to precipitate remains in solution at the point at which the second cation begins to precipitate? Show all work clearly for full credit.