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Exam 2-solutions

# Exam 2-solutions - Version 028 Exam 2 fakhreddine(51615...

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Version 028 – Exam 2 – fakhreddine – (51615) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points The equilibrium constant for the reaction 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) has the value K = 30000 at 700 K. Find the value of K for the reaction SO 3 (g) SO 2 (g) + 1 2 O 2 (g) at the same temperature. 1. 3 . 95 × 10 10 2. 6 . 32 × 10 6 3. 2 . 64 × 10 7 4. 8 . 23 × 10 8 5. 5 . 77 × 10 3 correct 6. 5 × 10 13 Explanation: For the first equation 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) K 1 = P 2 SO 3 P 2 SO 2 · P O 2 = 30000 . For the second reaction SO 3 (g) SO 2 (g) + 1 2 O 2 K = P SO 2 · P 1 / 2 O 2 P SO 3 = 1 K 1 = 1 30000 = 0 . 0057735 002 10.0points For gases initially contained in a 1.0 L vessel and the reaction 2 NO 2 (g) N 2 O 4 (g) Δ H = - 37 . 7 kJ/mol, which of the following would NOT cause the equilibrium to shift to the left and convert some N 2 O 4 to NO 2 ? I. addition of 1 mol He(g) to the vessel; II. isothermally decreasing the volume of the container to 0.425 L; III. addition of 1 mol of N 2 O 4 (g); IV. increasing the temperature; 1. IV only 2. II only 3. I and II correct 4. III only 5. II and IV 6. I and III 7. I only Explanation: 003 10.0points Assume that five weak acids, identified only by numbers (1, 2, 3, 4 and 5), have the follow- ing ionization constants. Ionization Acid Constant K a value 1 1 . 0 × 10 3 2 3 . 0 × 10 5 3 2 . 6 × 10 7 4 4 . 0 × 10 9 5 7 . 3 × 10 11 A 0.10 M solution of which acid would have the highest pH?

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Version 028 – Exam 2 – fakhreddine – (51615) 2 1. 1 2. 4 3. 2 4. 5 correct 5. 3 Explanation: K a = [H + ] [A ] [HA] HA H + + A A high pH means that the [H + ] concentra- tion is low. (Remember that values greater than 7 are basic!) The larger values of K a means that there is more [H + ] so you would expect these solutions to be more acidic; i.e. , have smaller pH’s. The smaller K a values mean less [H + ] in the solution, so higher pH’s. The acid with the smallest K a (#5) will have the highest pH and lowest [H + ] concentration. 004 10.0points Consider the following reaction: PCl 5 (g) PCl 3 (g) + Cl 2 (g) . At equilibrium, [PCl 5 ] = 2.00 M and [PCl 3 ] = [Cl 2 ] = 1.00 M. If suddenly 1.00 M PCl 3 (g) and Cl 2 (g) are added, calculate the new equi- librium concentration of PCl 5 (g). 1. 1.86 M 2. 2.17 M 3. 2.25 M 4. 2.08 M 5. 2.81 M correct 6. 1.53 M Explanation: K = (1)(1) 2 = 0 . 5 after the addition of 1 M for PCl 3 and Cl 2 the reaction must shift to the left to reestablish equilibrium. K = (2 - x ) 2 (2 + x ) = 0 . 5 which converts to 0 = x 2 - 4 . 5 x + 3 The quadratic formula will yield two an- swers for x . x = 0 . 8139 or 3 . 686 The 3.686 answer is not physically possible and therefore x = 0 . 8139. [PCl 5 ] = 2 + x = 2 . 81 M 005 10.0points A diprotic acid H 2 A has values of K a1 = 1 . 0 × 10 6 and K a2 = 1 . 0 × 10 10 . What is the [A 2 ] in a 0.10 M solution of H 2 A? 1. 1 . 0 × 10 10 M correct 2. 3 . 2 × 10 4 M 3. 3 . 2 × 10 6 M 4. 0.20 M 5. 0.10 M Explanation: 006 10.0points The pH of 0 . 015 M HNO 2 (nitrous acid) aque- ous solution was measured to be 2 . 63. What is the value of p K a of nitrous acid?
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