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solution7_pdf - ig996 Homework 7 Lyon(53565 This print-out...

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ig996 – Homework 7 – Lyon – (53565) 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. This homework is due Thursday, March 6 by 12:00pm. 001 10.0 points A lake in central Texas contains 1 billion liters of water. The pH is normally 6.0, but it is tested at 5.0 and the fish are in danger. How much CaCO 3 should be dumped in to bring the pH back to 6.0? Assume CaCO 3 + H + Ca 2+ + HCO 3 The molecular weight of CaCO 3 is 100.1 g/mol. 1. 900 kg correct 2. 157 kg 3. 90 kg 4. 900,000 kg 5. 157 g Explanation: V = 10 9 L pH normal = 6 . 0 pH current = 5.0 CaCO 3 = 100.1 g/mol [H 3 O + ] needed = [H 3 O + ] current - [H 3 O + ] normal = (10 5 . 0 - 10 6 . 0 ) M = 9 . 0 × 10 6 M CaCO 3 + H 3 O + Ca 2+ + HCO 3 + H 2 O n CaCO 3 = n H 3 O + = (10 9 L)(9 . 0 × 10 6 mol / L) = 9 . 0 × 10 3 mol m CaCO 3 = (9 . 0 × 10 3 mol)(100 . 1 g / mol) = 9 . 0 × 10 5 g = 900 kg 002 10.0 points The pH of 0 . 1 M HClO 2 (chlorous acid) aqueous solution was measured to be 1 . 2. What is the value of p K a for chlorous acid? 1. 2.57 2. 0.11 3. 0.96 correct 4. 1.20 5. 3.91 6. 1.40 Explanation: M = 0 . 1 M pH = 1 . 2 Analyzing the reaction with molarities, HClO 2 + H 2 O H 3 O + + ClO 2 0 . 1 - 0 0 - x - x x 0 . 1 - x - x x [H 3 O + ] = [ClO 2 ] = 10 pH = 10 1 . 2 = 0 . 0630957 mol / L The K a is K a = [H 3 O + ][ClO 2 ] [HClO 2 ] = (0 . 0630957) 2 0 . 1 - 0 . 0630957 = 0 . 107876 and the p K a is p K a = - log(0 . 107876) = 0 . 967077 . 003 10.0 points A solution is prepared by dissolving NaCl in water. The pH of this solution is closest to
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ig996 – Homework 7 – Lyon – (53565) 2 1. 11. 2. 7. correct 3. 5. 4. 9. 5. 3. Explanation: NaCl is a neutral salt. It does not alter pH. 004 10.0 points A 0.28 M solution of a weak acid is 3.5% ionized. What is the pH of the solution? 1. 0.55 2. 3.17 3. 2.01 correct 4. 5.25 5. 1.46 Explanation: M = 0 . 28 M P = 3 . 5% 3 . 5% of the 0 . 28 M is ionized (contributes to pH), so [H + ] = (0 . 28 M) × 3 . 5 100 = 0 . 0098 M pH = - log[H + ] = - log(0 . 0098) = 2 . 00877 005 10.0 points What is the H 3 O + concentration of a 0.15 M solution of NH 4 Cl in H 2 O at 25 C? ( K b for NH 3 = 1 . 8 × 10 5 ) 1. 9 . 1 × 10 6 M correct 2. 1 . 6 × 10 3 M 3. 1 . 1 × 10 9 M 4. 1 . 0 × 10 14 M 5. 4 . 9 × 10 5 M 6. 8 . 2 × 10 1 M Explanation: C a = 0 . 15 K w = 10 14 K b = 1 . 8 × 10 5 NH + 4 NH 3 + H + K a = K w K b = 10 14 1 . 8 × 10 5 = 5 . 56 × 10 10 Since NH + 4 is a weak acid, use the weak acid equation: [H + ] = radicalbig K a C a = radicalBig (5 . 56 × 10 10 )(0 . 15) = 9 . 13 × 10 6 M 006 10.0 points K a for HClO is 3 . 5 × 10 8 . What is the pH of a 0.3 M solution of HClO? You may approximate the full quadratic for problem because K c is so small. The reaction is HClO + H 2 O H 3 O + + ClO K w = 1 × 10 14 .
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