HW6 - ssh525 hw06 McCord (53580) 1 This print-out should...

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Unformatted text preview: ssh525 hw06 McCord (53580) 1 This print-out should have 15 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points Which of the five solutions below is most ba- sic? 1. 0.10 M CH 3 COOH and 0.10 M NaCH 3 COO 2. 0.10 M CH 3 COOH and 0.20 M NaCH 3 COO 3. 0.10 M aq NH 3 and 0.10 M NH 4 Cl cor- rect 4. 0.10 M HClO and 0.10 M NaClO 5. 0.10 M aq NH 3 and 0.20 M NH 4 Cl Explanation: All are buffer where pH = p K a + log parenleftbigg [base] [acid] parenrightbigg 002 10.0 points Carbonic acid (H 2 CO 3 ) is a diprotic acid with K a1 = 4 . 2 10- 7 and K a2 = 4 . 8 10- 11 . The ion product for water is K w = 1 . 10- 14 . What is the [HCO- 3 ] concentration in a 0.037 M H 2 CO 3 solution? 1. 4 . 8 10- 11 M 2. 3 . 7 10- 2 M 3. 1 . 6 10- 8 M 4. 4 . 2 10- 7 M 5. 1 . 2 10- 4 M correct Explanation: 003 10.0 points A 0.50 M solution of dimethylamine (a weak base with K b = 7 . 4 10- 4 ) would be resistant to large changes of pH Z1) when a strong acid is added, but not if a strong base is added. Z2) when a strong base is added, but not if a strong acid is added. Z3) when either a strong acid or a strong base is added. 1. only Z2) is true. 2. only Z1) is true. correct 3. under neither of the circumstances Z1) and Z2) 4. only Z3) is true. Explanation: M dimethylamine = 0.50 M K b = 7 . 4 10- 4 (CH 3 ) 2 NH + H 3 O + (CH 3 ) 2 NH + 2 + H 2 O The added acid reacts with the weak base to produce its conjugate acid. The presence in solution of appreciable amounts of the conju- gate pair produces a buffer. 004 10.0 points Explain why the salt of a weak acid, as well as the acid itself, must be present to form a buffer solution. 1. The anion from the salt is needed to partially neutralize added acid. correct 2. The anion from the salt is needed to partially neutralize added base. 3. The cation from the salt is needed to partially neutralize added base. 4. Actually, a weak acid by itself is a buffer; no salt is needed. 5. The cation from the salt is needed to partially neutralize added acid. Explanation: The salt of the acid provides the anion which is the conjugate base of the buffer sys- tem: HA + H 2 O A- + H 3 O + ssh525 hw06 McCord (53580) 2 This anion A- reacts with any added acid (H 3 O + ) to prevent any appreciable change in pH....
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This note was uploaded on 04/10/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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HW6 - ssh525 hw06 McCord (53580) 1 This print-out should...

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