All Notes, Books, Etc., Must Be Placed out of Sight. Please Read Each of the Problems Carefully s3

Chemistry 1212 1 Name______

Chemistry 1212

Exam III

Spring, 2001

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 the regular 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(Ca3(PO4)2 = 1.0 x 10-33 Kw = 1.0 x 10-14

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

Ka(HBen) = 6.6 x 10-5 Ka1(H2CO3) = 4.4 x 10-7

Ka(HAc) = 1.8 x 10-5 Ka2(H2CO3) = 4.7 x 10-11

Ka(HCN) = 5.8 x 10-10 Indicators: See separate Indicator Sheet

1.(15) A solution of sodium cyanide (NaCN) has a pH of 12.10. How many grams of NaCN (MW = 49.0) are in 425 ml of solution with the same pH? Show all of your work clearly for full credit.

2.(15) A student is asked to make an acetic acid—acetate buffer with a pH of 4.10. The only materials on hand are 300.0 ml of white vinegar which is 5.00% by mass of acetic acid (MW = 60), and potassium acetate (MW = 98.1). The white vinegar has a density of 1.0006 g/ml. How should the student construct this solution? (i.e. how many grams of KAc are needed?) Show all work clearly for full credit. (HINT: The molarity of a 5.00% white vinegar solution is 0.834 M HAc. Note that this problem without the hint would make an excellent example of an integrated final exam problem.)

3.(20) A solution with a pH of 8.73 is prepared by adding water to 0.614 mole of NaX to make 2.50 liters of solution. What is the pH of the solution after 0.219 mole of HX is added to the system? (Assume no volume change upon addition of the solid.) Show all of your work clearly for full credit.

4.(15) Calculate the pH and [CO3-2] of 0.63 M H2CO3 solution. Show all work clearly for full credit.

5.(20) 25.00 ml of 0.175 M solution of benzoic acid, HBen, is titrated with 0.469 M KOH. (Assume volumes are additive.) Show all work clearly for full credit.

a) Write a balanced equation for the titration reaction.

b) What is the pH of the solution before any potassium hydroxide is added?

c) What volume of potassium hydroxide is required to reach the equivalence point?

d) What is the pH of the solution at the equivalence point?

e) Select an appropriate indicator from the separate Indicator Sheet.

6.(15) Calcium nitrate is added to 0.15 M sodium phosphate.

a) At what concentration of Ca+2 does a precipitate first start to form? Show all work for full credit.

b) What is [Ca+2] when only 15% of the original PO4-3 remains? Show all work for full credit.

Chemistry Extra Credit: I have become convinced that some people are working together on the extra credit questions. Therefore, I offer one here to be done with the exam if you have time, but there is no take-home extra credit opportunity.

Which of the following is/are TRUE regarding a 0.10 M solution of a weak base, B¯?

a) [HB] = 0.10 M ______b) [OH¯]  [HB] ______

c) [B¯] > [HB] ______d) [H+] = ______

e) pH = 13.0 ______