Chemistry 65.100 A and V
Fourth Test
March 5, 1999

Part A. Answer all questions (5 marks each)

1. Referring to the phase diagram for water shown below, identify the phases present at points A, B, C and D. Name points E and F. What are the temperatures (in degrees C) at points G and H?

2. Why is the boiling point of water increased upon addition of a non-volatile solute?

3. Will an aqueous solution of KF be acidic, basic or neutral? Why? (Ka for HF is 3.5 x 10-4. Kb for KOH is very large.)

4. Why is HClO a weaker acid than HClO2?

5. For the reaction HNO3 + H2O ¾ H3O+ + NO3-, identify the two bases and state which one is stronger and how you know. (Ka for HNO3 is 30.)

6. Would the solubility of Cu3(PO4)2(s) increase or decrease as the pH is lowered? Why? (H3PO4(aq) is a weak acid.)

 

Part B. Answer one of the following two questions. (30 marks). If you answer both, the best one will be used to calculate your mark for part B.

1. A buffer solution containing 3.0 M acetic acid and an unknown concentration of sodium acetate has a pH of 3.74. The acid dissociation constant of acetic acid is 1.8 x 10-5.

(a) Use the Henderson-Hasselbach equation to calculate the concentration of sodium acetate.

(b) 0.05 mol of NaOH are added to 1 L of the buffer solution. Calculate the resultant pH.

2. Calculate the pH and concentrations of all species present (except water) in a 2 M aqueous solution of selenious acid, H2SeO3. For this acid, Ka1 = 0.035 and Ka2 = 5.0 x 10-8.

Part C. Answer one of the following two questions. (30 marks). If you answer both, the best one will be used to calculate your mark for part C.

3. (a) Calculate the freezing AND boiling points (oC) of a solution made by dissolving 1.00 mole of Na2SO4(s) in enough water to make 1.00 L of solution. The density of the solution is 1.12 g/mL. You can not assume that molality = molarity in this problem.

(b) Calculate the vapor pressure above the solution at 25oC. The vapor pressure of pure water at 25oC is 23.8 mm Hg. Note that Na2SO4(aq) is an involatile solute.

4. (a) The Ksp for lead chloride, PbCl2(s), is 1.2 x 10-5. Calculate the lead ion concentration (mol L-1) in a saturated aqueous solution of PbCl2.

(b) NaCl is added to the above solution until [Cl-] = 1.0 mol L-1. Calculate the concentration of lead ions in this solution.

Some Useful Constants

R = 0.082 L atm K-1 mol-1 = 8.314 J K-1mol-1

Kf (water) = 1.86 oC kg mol-1 Kb (water) = 0.51 oC kg mol-1