# chexam2 - Version 099 Exam 2 Holcombe(52460 This print-out...

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Version 099 – Exam 2 – Holcombe – (52460) 1 This print-out should have 33 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. Some constants: Kw=1e-14; R=8.314 J/K- mol 001 10.0 points Consider the fractional composition diagram for phosphoric acid. 0.2 0.4 0.6 0.8 1.0 2 4 6 8 10 12 14 pH Fractional Composition What is the approximate pH of a solu- tion prepared by adding several grams of Na 2 HPO 4 to a few hundred mL of water? 1. 7 2. 2 3. 5 4. 12 5. 10 correct Explanation: Na 2 HPO 4 is the salt of the monoprotic species which is the dominate species between pH’s of 7.2 and 12.7 which is the orange or shortest dashed curve. The best estimate of the pH is the dead center of that range which is a pH of 10 which is the average of p K 2 and p K 3 for the acid. 002 10.0 points A solution containing an unspecified amount of HCl and ammonia (NH 3 ) has a pH of 8 . 55 What is the ratio of NH 3 to NH + 4 in the solution? (For NH 3 , K b = 1 . 8 × 10 5 ) 1. 0 . 128 2. 5 . 08 3. 0 . 197 correct 4. 3 . 76 5. 0 . 266 6. 5 . 6 × 10 10 7. cannot be determined Explanation: Consider this equilibria... NH + 4 NH 3 + H + Rearranging the equilibrium expression gives... K a [H + ] = [NH3] [NH4+] = ratio K a = K w / 1 . 8 × 10 5 = 5 . 556 × 10 10 [H+] = 10 8 . 55 = 2 . 81838 × 10 9 M ratio = 5 . 556 × 10 10 2 . 81838 × 10 9 M = 0 . 197 Note that the same answer could be ob- tained by using the base equilibria and K b 003 10.0 points The pH of a solution changes from 5.5 to 3.5. This indicates that the [OH ] of the solution has 1. decreased by 2 M. 2. decreased by a factor of 100. correct 3. decreased by a factor of 2. 4. increased by a factor of 100. 5. increased by 2 M. 6. increased by a factor of 2. Explanation: 004 10.0 points

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Version 099 – Exam 2 – Holcombe – (52460) 2 The conjugate acid of HCO 3 is ? and the conjugate base of HCO 3 is ? . 1. H 2 CO 3 ; H 2 O 2. H + ; OH 3. H + ; CO 2 3 4. CO 2 3 ; H 2 CO 3 5. H 2 CO 3 ; CO 2 3 correct 6. CO 2 3 ; OH 7. CO 2 3 ; CO 2 3 8. H 2 CO 3 ; H 2 CO 3 Explanation: To produce a conjugate acid, HCO 3 must be acting as a base. Therefore it is a proton acceptor forming H 2 CO 3 . To produce a conjugate base, it must be acting as an acid. Therefore it is a proton donator forming CO 2 3 . 005 10.0 points A water solution of NaI will exhibit a pH value of 1. about 7. correct 2. less than 7. 3. zero. 4. greater than 7. Explanation: 006 10.0 points Based on its dot structure, the best classifica- tion for O 2 is as 1. an Arrhenius base. 2. a Lewis base. correct 3. an Arrhenius acid. 4. a Lewis acid. 5. a Bronsted Lowry acid. Explanation: The oxygen anion has a complete octet as its dot structure shows. Since it can only make available a share in an electron pair, it must be a Lewis base. 007 10.0 points What is the pH of a solution made by dissolving 0.100 mole of sodium acetate (NaCH 3 COO) in enough water to make one liter of solution? K a for CH 3 COOH is 1 . 80 × 10 5 .
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