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exam 2 ch 302 - Version 061 Exam 2 Holcombe(52460 This...

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Version 061 – 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 Sodium fluoride is added to pure water and stirred to dissolve. Compared to pure water, the new solution is 1. unchanged pH 2. neutral pH 3. basic pH correct 4. More than one of these is true. 5. acidic pH Explanation: Sodium fluoride is a basic salt and therefore changes solutions to a more basic pH. 002 10.0 points How many grams of HNO 3 are needed to make 315 mL of a solution with a pH of 2.60? The molar mass of HNO 3 is 63.02 g/mol. 1. 19.9 g 2. 1 . 25 × 10 5 g 3. 0.158 g 4. 0.0498 g correct 5. 0.502 g Explanation: pH = 2 . 6 V = 315 mL = 0 . 315 L MM = 63 . 02 g/mol [H + ] = 10 pH = 10 2 . 6 = 0 . 00251189 Since HNO 3 is a strong acid, HNO 3 (aq) H + (aq) + NO 3 (aq) . One HNO 3 one H + , so [H + ] = [HNO 3 ] = 0 . 00251189 mol and (0 . 315 L) × 0 . 00251189 mol HNO 3 1 L = 0 . 000791244 mol HNO 3 . Thus (0 . 000791244 mol HNO 3 ) × 63 . 02 g HNO 3 1 mol HNO 3 = 0 . 0498642 g HNO 3 003 10.0 points What would be the pH of a 10 9 M solution of HCl? 1. 6.998 correct 2. 6.921 3. 6.954 4. 6.876 Explanation: For a dilute strong acid, [H + ] 2 - Ca[H + ] - K w = 0 [H + ] = x = parenleftBigg - b ± b 2 - 4ac 2a parenrightBigg pH = 6.998 004 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. C H 2 CO 3 = [HCO 3 ] + [CO 2 3 ] 2. [H + ] = [HCO 3 ] + [CO 2 3 ] + [OH ] 3. C H 2 CO 3 = [H 2 CO 3 ] + [HCO 3 ] + [CO 2 3 ] correct 4. K w = [H + ] + [OH ] Explanation: 005 10.0 points A solution at a pH of 1.0 contains the weak acid HA and its conjugate base A . What
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Version 061 – Exam 2 – Holcombe – (52460) 2 percent of the acid is dissociated? ( K a = 10 6 and [HA] + [A ] = 10 3 M). 1. Not enough information is given. 2. 100% 3. between 10% and 90% 4. < 10% correct 5. between 90% and 100% Explanation: 006 10.0 points What would be the pH of a 1 M solution of Na 2 HPO 4 ? Assume H 3 PO 4 has a p K a 1 of 2 and a p K a 2 of 7 and a p K a 3 of 13. 1. 13 2. 2 3. 7.5 4. 10 correct 5. 4.5 6. 7 Explanation: For a solution composed of a single ampho- teric species (HPO 4 2 ), pH = 0 . 5(p K a 2 + p K a 3 ) = 0 . 5(7 + 13) = 10 007 10.0 points What is the concentration of SO 2 4 in 2.0 M H 2 SO 4 ? K a1 is strong and K a2 = 1 . 2 × 10 2 . 1. 1 . 0 × 10 7 M 2. 4 . 0 × 10 1 M 3. 2 . 0 × 10 1 M 4. 4 . 0 × 10 2 M 5. 1 . 2 × 10 2 M correct Explanation: 008 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. None of these 2. pH = 9.26 correct 3. pH = 7.12 4. pH = 4.74 5. pH = 3.63 Explanation: [base] = 0.05 M As mentioned, this is a weak base, so use the equation to calculate weak base [OH ] concentration: [OH ] = radicalbig K b C b = radicalBig (6 . 6 × 10 9 ) (0 . 05) = 1 . 81659 × 10 5 After finding [OH ], you can find 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 009 10.0 points
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Version 061 – Exam 2 – Holcombe – (52460) 3 A water solution of NaI will exhibit a pH value of 1. less than 7.
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