Homework 6-solutions - rathbun(rar2954 Homework 6 vanden...

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rathbun (rar2954) – Homework 6 – vanden bout – (51640) 1 This print-out should have 16 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points For a weak diprotic acid system (H 2 A) which of the following statements are true? I) H 2 A and A 2 - form a bu ff er. II) K a1 > K a2 III) [A 2 - ] > [HA - ] in acidic solution IV) HA - is amphiprotic. 1. I and III only 2. I and IV only 3. II and IV only correct 4. II only Explanation: H 2 A and HA - or HA - and A 2 - will form bu ff ers, but H 2 A and A 2 - will not form a bu ff er. For polyprotic acids, protons become harder to remove (dissociate) as the ion be- comes more charged. Therefore K a1 > K a2 for this system. In acidic solution (access pro- tons) all ions will be protonated. HA - is amphiprotic because it can accept a proton or it can donate a proton. 002 10.0 points Which of the equilibrium expressions for a triprotic acid H 3 A would be involved in the calculation to find the pH of a solution found from LiCaA and Na 2 HA? Assume the K values are far apart and K w is not involved in the calculation. 1. K a1 2. K a2 and K a3 3. K a1 and K a2 4. K a2 5. K a1 , K a2 , and K a3 6. K a3 correct Explanation: The salts use HA 2 - and A 3 - , so K a3 is needed. 003 10.0 points What is the sulfide ion concentration [S 2 - ] in a saturated solution of H 2 S at pH = 4? [H 2 S] = 0.10 M, K a1 × K a2 = 1 . 1 × 10 - 20 for H 2 S. 1. 0.10 M 2. 3 . 3 × 10 - 7 M 3. 1 . 0 × 10 - 4 M 4. 1 . 1 × 10 - 17 M 5. 1 . 1 × 10 - 13 M correct Explanation: 004 10.0 points How many amphoteric (amphiprotic) species is a tetraprotic acid capable of pro- ducing? 1. 2 2. 1 3. 4 4. 5 5. 3 correct Explanation: The product of the first 3 deprotonation events will result in an amphoteric species.
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