Chem exam 2

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Version 183 – Exam 2 – Holcombe – (52460) 1 This print-out should have 33 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. Some constants: Kw=1e-14; R=8.314 J/K- mol 001 10.0 points A water solution oF NaI will exhibit a pH value oF 1. less than 7. 2. zero. 3. about 7. correct 4. greater than 7. Explanation: 002 10.0 points Complete the Following statement. A pH value oF - 1 . 5 1. is impossible because pH cannot be nega- tive. 2. indicates an H + concentration oF - 0 . 0176 M. 3. indicates an H + concentration oF 0.176 M. 4. indicates an H + concentration oF 0.0316 M. 5. indicates an H + concentration oF 31.6 M. correct 6. indicates an H + concentration oF - 0 . 0316 M. Explanation: [H + ] = 10 pH = 10 ( 1 . 5) = 31 . 6228 M pH CAN be negative! 003 10.0 points When dissolved in water, bases produce 1. hydronium ions. 2. H 3 O + ions. 3. OH ions. correct 4. salts. 5. H + ions. Explanation: Bases produce hydroxide ions (OH ) in wa- ter; acids produce hydronium ions (H 3 O + ). 004 10.0 points Select the solution with the lowest pH 1. 10 3 M, acetic acid. 2. 10 3 M, NaOH. 3. 10 5 M, acetic acid. 4. 10 3 M, HCl. correct 5. 10 3 M, Ca(OH) 2 . 6. 10 5 M, HCl. Explanation: HCl is the only strong acid so all the HCl present will produce H 3 O + . 10 3 M HCl has a higher concentration than 10 5 M HCl. 005 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 ±ractional Composition
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Version 183 – Exam 2 – Holcombe – (52460) 2 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. 5 2. 7 3. 2 4. 10 correct 5. 12 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. 006 10.0 points A solution at a pH of 1.0 contains the weak acid HA and its conjugate base A . What percent of the acid is dissociated? ( K a = 10 6 and [HA] + [A ] = 10 3 M). 1. Not enough information is given. 2. < 10% correct 3. between 10% and 90% 4. 100% 5. between 90% and 100% Explanation: 007 10.0 points The conjugate acid of HCO 3 is ? and the conjugate base of HCO 3 is ? . 1. CO 2 3 ; H 2 CO 3 2. H + ; CO 2 3 3. H 2 CO 3 ; CO 2 3 correct 4. H + ; OH 5. CO 2 3 ; CO 2 3 6. H 2 CO 3 ; H 2 O 7. CO 2 3 ; OH 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
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