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solutionexam3_pdf - Version 158 – Exam 3 – Sutcliffe...

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Unformatted text preview: Version 158 – Exam 3 – Sutcliffe – (52410) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. NOTE: The values of K a for acetic acid and K b for ammonia may be found in a different question; they are used in several questions but not quoted in all of them. 001 10.0 points What is the pH of a 0 . 2 M solution of Ca(OH) 2 ? Assume complete dissociation of the base. 1. 10.8 2. 13.6 correct 3. 0.4 4. 7 Explanation: [Ca(OH) 2 ] = 0.2 M Ca(OH) 2 is a strong base which completely dissociates. Ca(OH) 2 −→ Ca 2+ + 2 OH − [OH − ] = 2 [Ca(OH) 2 ] = 2 (0 . 2 M) = 0 . 4 M pH = 14 − pOH = 14 − log 0 . 4 = 13 . 6 002 10.0 points A 0.10 M solution of the weak acid HClO con- tains 5.9 × 10 − 5 moles of H 3 O + ions per liter. What is the ionization constant of HClO? 1. 3 . 5 × 10 − 8 correct 2. 3 . 5 × 10 − 9 3. 5 . 9 × 10 − 4 4. 3 . 5 × 10 − 10 5. 5 . 9 × 10 − 5 Explanation: 003 10.0 points What is the buffer capacity of 50 mL of 0.3 M HNO 2 and 100 mL of 0.5 M NaNO 2 ? 1. 0.05 mol of OH − and 0.015 mol of H + 2. 0.15 mol of OH − and 0.5 mol of H + 3. 0.5 mol of OH − and 0.3 mol of H + 4. 0.3 mol of OH − and 0.5 mol of H + 5. 0.015 mol of OH − and 0.05 mol of H + correct Explanation: V 1 = 50 mL M 1 = 0 . 3 M V 2 = 100 mL M 2 = 0 . 5 M HA ⇀ ↽ A − + H + 50 mL 100 mL 0.3 M 0.5 M 15 mmol 50 mmol Adding a strong base will introduce OH − ; the base will react with HA. There is only 0.015 mol of HA so only 0.015 mol of OH − can be added before the buffer capacity is exceeded. Adding a strong acid introduces H + ; the acid will react with A − . There is only 0.05 mol of A − so only 0.05 mol of H + can be added before exceeding buffer capacity. 004 10.0 points For a given weak acid HA the value of K a 1. does not change with temperature. 2. cannot be less than 10 − 7 . 3. changes with the pH of the solution. 4. changes if we add a strong acid to a solution of the weak acid. 5. changes if we add a strong base to a solution of the weak acid. 6. None of these correct 7. cannot be greater than 10 − 7 . Version 158 – Exam 3 – Sutcliffe – (52410) 2 Explanation: 005 10.0 points What is the pH of an aqueous solution that is 0.60 M (CH 3 ) 3 N ( K b = 6 . 5 × 10 − 5 ) and 0.95 M (CH 3 ) 3 NHCl? 1. 3.99 2. 9.61 correct 3. 10.01 4. 9.81 5. 4.39 Explanation: 006 10.0 points What is the approximate pH of a solution in which the concentration of nitrous acid is 0 . 1 M and the concentration of sodium nitrite is . 8 M? K a for nitrous acid is 4 . 5 × 10 − 4 . 1. 2 . 89 2. 5 . 9 3. 6 . 55 4. 4 . 25 correct 5. 1 . 78 Explanation: [NO − 2 ] = 0 . 8 M K a = 4 . 5 × 10 − 4 [HNO 2 ] = 0 . 1 M HNO 2 → H + + NO − 2 pH = p K a + log parenleftbigg [NO − 2 ] [HNO 2 ] parenrightbigg = − log(4 . 5 × 10 − 4 ) + log parenleftbigg . 8 . 1 parenrightbigg = 4 . 25 007 10.0 points How many grams of sodium acetate must be...
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solutionexam3_pdf - Version 158 – Exam 3 – Sutcliffe...

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