# exam 2 - Version 254 – Exam 2 – Holcombe –(52460 This...

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Version 254 – 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 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. between 10% and 90% 2. < 10% correct 3. 100% 4. between 90% and 100% 5. Not enough inFormation is given. Explanation: 002 10.0 points Select the solution with the lowest pH 1. 10 3 M, NaOH. 2. 10 3 M, acetic acid. 3. 10 5 M, HCl. 4. 10 3 M, Ca(OH) 2 . 5. 10 3 M, HCl. correct 6. 10 5 M, acetic acid. 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. 003 10.0 points You have a weak molecular base with K b = 6 . 6 × 10 9 . What is the pH oF a 0.0500 M solution oF this weak base? 1. pH = 3.63 2. None oF these 3. pH = 7.12 4. pH = 4.74 5. pH = 9.26 correct Explanation: [base] = 0.05 M As mentioned, this is a weak base, so use the equation to calculate weak base [OH ] concentration: [OH ] = r K b C b = R (6 . 6 × 10 9 ) (0 . 05) = 1 . 81659 × 10 5 AFter fnding [OH ], you can fnd pH using either method below: A) pOH = - log ( 1 . 81659 × 10 5 ) = 4 . 74074 pH = 14 - 4 . 74074 = 9 . 25926 or B) K w = [H + ][OH ] = 1 × 10 14 [H + ] = K w [OH ] = 1 × 10 14 1 . 81659 × 10 5 = 5 . 50482 × 10 10 pH = - log ( 5 . 50482 × 10 10 ) = 9 . 25926 004 10.0 points Would the value oF K w change iF the temper- ature were raised From 25 C to 50 C? 1. No 2. Yes correct Explanation: 005 10.0 points A solution oF 0.2 M boric acid is prepared as

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Version 254 – Exam 2 – Holcombe – (52460) 2 an eye wash. What is the approximate pH of this solution? For boric acid K a = 7 . 2 × 10 10 . 1. pH = 5 correct 2. pH = 4 3. pH = 7 4. pH = 3 5. pH = 6 Explanation: 006 10.0 points The conjugate base of H 2 SO 4 is: 1. H 2 O 2. H 2 SO 3 3. H 3 O + 4. SO 2 4 5. OH 6. HSO 4 correct 7. H 3 SO + 4 Explanation: Remove one H + ion from a species to ±nd its conjugate base. 007 10.0 points Which of the following is a correct mass bal- ance expression for the addition of H 2 CO 3 to water? 1. K w = [H + ] + [OH ] 2. C H 2 CO 3 = [H 2 CO 3 ] + [HCO 3 ] + [CO 2 3 ] correct 3. [H + ] = [HCO 3 ] + [CO 2 3 ] + [OH ] 4. C H 2 CO 3 = [HCO 3 ] + [CO 2 3 ] Explanation: 008 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 . 1.
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## This note was uploaded on 09/14/2010 for the course C E 301 taught by Professor Staff during the Spring '08 term at University of Texas.

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exam 2 - Version 254 – Exam 2 – Holcombe –(52460 This...

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