Chemistry 65.100 A and V
Fourth Test
March 6, 1998

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. What is the temperature at point E?

2. Express Raoult’s law for a non-volatile solute and explain each term in the equation.

3. Will an aqueous solution of NH₄ClO₄ be acidic, basic or neutral? Why? K_a for HClO₄ is very large. K_b for NH₃ is 1.8 x 10^-5.

4. Why is HOBr a weaker acid than HOCl?

5. K_a for HF is 6.8 x 10^-4. K_a for HCl is 10⁶. Which is the stronger base: F^- or Cl^-? How do you know?

6. Copper hydroxide, Cu(OH)_2(s) is slightly soluble in water. Will it be more or less soluble in an acidic solution. Why?

Part B. Answer all questions (20 marks each)

1. Calculate the pH and the concentrations of all species present (i.e. H₂CO₃, HCO₃^- and CO₃^-2) in an aqueous solution in equilibrium with CO_2(g) at 3 atm pressure. For CO_2(g), K_H = 0.032 mol L^-1 atm^-1. For H₂CO_3(aq), K_a1 = 4.4 x 10^-7 and K_a2 = 5.6 x 10^-11.

2. (a) The K_sp for strontium fluoride, SrF₂, is 2.5 x 10^-9. Calculate the strontium ion concentration (mol L^-1) in a saturated aqueous solution of SrF₂.

(b) NaF_(s) is added to the above solution until [NaF_(aq)] = 0.5 mol L^-1. Calculate the new concentration of strontium ions.

(c) Calculate the boiling point (^oC) of the solution in part (b). You may assume that the density of the solution is the same as that of pure water. K_b for water is 0.51 ^oC kg mol^-1.

(d) Calculate the osmotic pressure (atm) of the solution of the solution in part (b) at 25⁰C.

3. At 34.1^oC, the vapor pressure of pure water is 40.1 mm Hg. DH_vap for water is 40.7 kJ mol^-1. Calculate the vapor pressure of water at 85.5^oC.

Some Useful Constants

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