exam 2 ch 302 - Version 061 – Exam 2 – Holcombe –...

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Unformatted text preview: 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) × . 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 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 . × 10 − 7 M 2. 4 . × 10 − 1 M 3. 2 . × 10 − 1 M 4. 4 . × 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?...
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This note was uploaded on 04/14/2010 for the course CH CH302 taught by Professor Mccord during the Spring '10 term at University of Texas at Dallas, Richardson.

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exam 2 ch 302 - Version 061 – Exam 2 – Holcombe –...

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