CH 302 - [McCord] [Spring 2010] - Test 2

# CH 302 - [McCord] [Spring 2010] - Test 2 - Version 088 –...

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Unformatted text preview: Version 088 – Exam 2 – Mccord – (52450) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord CH302 1pm This exam is only for McCord’s MWF 1pm CH302 class. 001 10.0 points The equilibrium constant for the reaction 2 SO 2 (g) + O 2 (g) ⇀ ↽ 2 SO 3 (g) has the value K = 4 × 10 24 at 298 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. 5 × 10 − 13 correct 2. 5 . 77 × 10 − 3 3. 3 . 95 × 10 − 10 4. 2 . 64 × 10 − 7 5. 6 . 32 × 10 − 6 6. 8 . 23 × 10 − 8 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 = 4 × 10 24 . 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 √ 4 × 10 24 = 5 × 10 − 13 002 10.0 points A solution of HClO is mixed and found to have a pH of 4 . 74. Find what the initial concentration of HClO was for this solution. K a = 3 . 00 × 10 − 8 for HClO. 1. 0.10072 2. 0.191889 3. 0.0110559 4. 0.210399 5. 0.0553603 6. 0.0290616 7. 0.481933 8. 0.139021 9. 0.021057 10. 0.0220488 Correct answer: 0 . 0110559 mol / L. Explanation: pH = 4 . 74 K a = 3 × 10 − 8 At equilibrium, [H 3 O + ] = 10 − pH = 10 − 4 . 74 = 1 . 8197 × 10 − 5 mol / L . Analizing the reaction using concentra- tions, HClO + H 2 O ⇀ ↽ H 3 O + + ClO − C − − x − x x C − x − x x x = [H 3 O + ], so K a = x 2 C − x C − x = x 2 K a Version 088 – Exam 2 – Mccord – (52450) 2 C = x 2 K a + x = (1 . 8197 × 10 − 5 ) 2 3 × 10 − 8 + 1 . 8197 × 10 − 5 = 0 . 0110559 mol / L . 003 10.0 points Auto-protolysis of water is the same as the autoionization of water. Consider a glass of water at 80 ◦ C, which of the following statements is true? 1. The water is acidic because its pH is slightly lower than 7 2. The water is neutral because its pH is 7 3. The concentrations of H + and OH − are equal correct 4. The auto-protolysis of water is an exothermic process Explanation: The auto-protolysis of water is an endother- mic process since energy is needed to break the bonds. At a higher temperature, more H + ions exist due to higher energy, thus the pH of water decreases. The water is not acidic be- cause pH = pOH, although the K w is not 14 in this case. 004 10.0 points Rank following acids from most to least acidic: hydrocyanic acid (HCN) K a = 6 . 2 × 10 − 10 hypoiodous acid (HOI) K a = 2 × 10 − 11 chlorous acid (HClO 2 ) K a = 1 . 2 × 10 − 2 acetic acid (CH 3 COOH) K a = 1 . 8 × 10 − 5 1. HOI > HClO 2 > CH 3 COOH > HCN 2. CH 3 COOH > HCN > HOI > HClO 2 3. HCN > HOI > HClO 2 > CH 3 COOH 4. HClO 2 > CH 3 COOH > HCN > HOI correct Explanation: Explanation: The value of K a is directly proportional to the acidity (strength) of the acid. By ranking from the largest (1 . 2 × 10 − 2 ) to the smallest (2 × 10 − 11 ) Ka, one is ranking from the most to least acidic.from the most to least acidic....
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## This note was uploaded on 11/18/2010 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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CH 302 - [McCord] [Spring 2010] - Test 2 - Version 088 –...

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