UNIVERSITY OF TORONTO

FACULTY OF APPLIED SCIENCE AND ENGINEERING

FINAL EXAMINATION - APRIL 2002

First Year - Elective

CHE112H1 - PHYSICAL CHEMISTRY

Examiner - C.E. Chaffey

No aids. A non-programmable calculator may be used.
Answser all six questions, which have values as indicated; total = 100.
R = 8.314471 J K^-1 mol^-1.     F = 96485 C mol^-1.
1 L = 10^-3 m³.     T/K = .      P° = 1 bar = 10⁵ Pa.

1. A building has two rooms, with volues 40 m³ and 100 m³. Initially in both rooms the air pressure is 97 kPa and the temperature is 20°C. Then a fan located between the rooms is operated to blow air into the smaller room, and a vent allows air to return to the larger room. In the steady state the pressure in the smaller room is 7.0 kPa higher than in the larger room, and the temperature is still uniform at 20°C. No air enters or leaves the building.
(5) a. What is the mass of the air in the building?
(5) b. While the fan is operating, what is the (absolute) pressure P of the air in the smaller room?
(5) c. What is an average speed of a nitrogen molecule in the air in the smaller room (any kind of average)?
Data: Air is a mixture of nitrogen, N₂ (28.0 g mol^-1), oxygen, O₂ (32.0 g mol^-1), and argon, Ar (40.0 g mol^-1). The mole fractions of O₂ and Ar are 0.21 and 0.01 respectively.

2. In an electrochemical cell, one electrode is metallic lead, Pb, and the other electrode is metallic silver, Ag, completely coated with insoluble solid silver bromide, AgBr. Both electrodes are put into a single electrolyte, a 0.0100 mol L^-1 solution in water of lead bromide, PbBr₂, and they are connected by a voltmeter having a high resistance.
(14) a. What is the cell potential E_cell at 25°C?
(6) b. [i] Which electrode is positive? [ii] If the cell operates as a galvanic cell, which electrode is the anode? [iii] Why is it not necessary to have the electrodes in separate compartments connected by a salt bridge?
Data: Standard reduction potentials at 25°C.
Pb²⁺ + 2e^- = Pb, E° = -0.1262 V.          AgBr + e^- = Ag + Br^-, E° = +0.0713 V.

3. For an optical application, a deep orange liquid solution is made by dissolving 1.00 kg of trans-para-azotoluene, C₁₄H₁₄N₂ (210.3 g mol^-1), a non-volatile colored solute, in 10.0 kg of n-pentane, C₅H₁₂ (72.15 g mol^-1).
(10) What is the standard boiling point of the solution (at P° = 1 bar)?
Data for C₅H₁₂:

Vapor pressure at 25°C, P•, kPa	68.67
Molar enthalpy of vaporization, hvap, kJ mol-1	25.79
Boiling point elevation constant, Kb, K kg mol-1	2.22

4. In an excellent answer to this question, any approximations will be shown to be valid. Formic acid, HCOOH, occurs naturally in the bodies of ants. It is a weak acid, with Ka = 1.7×10^-4 at 25°C. 2.00 mol of formic acid is dissolved in enough water to make 25.0 L of solution. At 25°C:
(6) a. What is the pH of this solution?
(8) b. [i] What volue of 0.250 mol L^-1 sodium hydroxide, NaOH, solution must be added, in order to add an amount of base stoichimetrically equivalent to the acid? This amount corresponds to the end (or equivalence) point of a titration. [ii] What is the pH of the resulting solution?
(6) c. When only one-half of the volue of sodium hydroxide solution needed in part b has been added to the original formic acid solution, what is the pH?

5. Hypochlorite ion, OCl^-, a moderate oxidizing agent useful as a disinfectant, can oxidize iodide, I^-, to hypoiodite, OI^-: OCl^- + I^- = OI^- + Cl^-. At time t = 0, 0.00100 mol of solid potassium iodide, KI, is quickly dissolved in 0.500 L of a solution containing hypochlorite with [OCl^-] = 0.00200 mol L^-1. The concentration [OCl^-] at later times is tabulated. (The temperature is constant at 25°C.)

t, s	120	360	600	780
[OCl-], mol L-1	0.00160	0.00114	0.00088	0.00076