Exam2_Solutions

# Exam2_Solutions - Version 129 – Exam 2 – Sutcliffe –...

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Unformatted text preview: Version 129 – Exam 2 – Sutcliffe – (52410) 1 This print-out should have 28 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Consider the reaction 3 Fe(s) + 4 H 2 O(g) → 4 H 2 (g) + Fe 3 O 4 (s) . If the volume of the container is reduced, 1. more Fe(s) is produced. 2. no change occurs. correct 3. more H 2 (g) is produced. 4. more H 2 O(g) is produced. 5. the equilibrium constant increases. Explanation: 002 10.0 points Assume you have 50g of each compound. The vapor pressure of pure CH 2 Cl 2 (with molecular weight 85 g/mol) is 133 torr at 0 ◦ C and the vapor pressure of pure CH 2 Br 2 (with molecular weight 174 g/mol) is 11 torr at the same temperature. What is the total vapor pressure at 0 ◦ C of a solution prepared from equal masses of these two substances? 1. 89 torr 2. 7.4 torr 3. 124 torr 4. 72 torr 5. 105 torr 6. 44 torr 7. 93 torr correct 8. 144 torr 9. 3.6 torr Explanation: For CH 2 Cl 2 , P = 133 torr MW = 85 g/mol For CH 2 Br 2 , P = 11 torr MW = 174 g/mol The mole fractions are X CH 2 Cl 2 = n CH 2 Cl 2 n CH 2 Cl 2 + n CH 2 Cl 2 = g / 85 g / 85 + g / 174 = 0 . 672 X CH 2 Br 2 = n CH 2 Br 2 n CH 2 Br 2 + n CH 2 Cl 2 = g / 174 g / 174 + g / 85 = 0 . 328 Thus P solution = P CH 2 Cl 2 + P CH 2 Br 2 = X CH 2 Cl 2 P CH 2 Cl 2 + X CH 2 Br 2 P CH 2 Br 2 = (0 . 672)(133 torr) + (0 . 328)(11 torr) = 93 . 0 torr 003 10.0 points A dissolution process is exothermic if the amount of energy released in bringing about ? interactions is greater than the sum of the amounts of energy absorbed in overcom- ing ? and ? interactions. 1. solute-solute; crystal lattice; solvent- solvent 2. solute-solute; solvent-solvent; solvent- solute 3. solvent-solute; solute-solute; solvent- solvent correct 4. solvent-solvent; solute-solute; solvent- solute 5. solvent-solute; solute-solute; crystal lat- tice Version 129 – Exam 2 – Sutcliffe – (52410) 2 Explanation: In the course of dissolution, you disrupt solute-solute interactions and solvent-solvent interactions in order to create new solvent- solute interactions. So you can imagine that you have to invest energy to break up solute- solute interactions and solvent-solvent inter- actions. However, you get energy back in the creation of solvent-solute interactions. If your investment in solute-solute and solvent- solvent interactions is less than your return in solvent-solute interactions, excess energy (as heat) will be produced and the reaction will be exothermic. 004 10.0 points Which of Acid X [HX] = 10 − 3 M K a = 1 . 32 × 10 − 6 Acid Y [HY] = 10 − 3 M K a = 3 . 47 × 10 − 3 is the stronger acid? 1. Acid Y is stronger. correct 2. They have the same strength....
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## This note was uploaded on 03/03/2012 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas.

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Exam2_Solutions - Version 129 – Exam 2 – Sutcliffe –...

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