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exam2 - Version 067 – Exam 2 – Fakhreddine –(52395...

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Version 067 – Exam 2 – Fakhreddine – (52395) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points How much acetic acid ( K a = 1 . 8 × 10 5 ) must be used to prepare a liter of solution having a pH of 2.75? 1. 0.178 moles correct 2. 0.435 moles 3. 5.69 moles 4. 0.0101 moles 5. 9.88 moles Explanation: pH = log[H + ] = 2 . 75 so [H + ] = log 1 ( 2 . 75) = 1 . 778 × 10 3 mol HA H + + A Substitute [H + ] = [A ] = 1 . 778 × 10 3 into the equation for K a : K a = [H + ][A ] [HA] [HA] = [H + ][A ] K a = ( 1 . 778 × 10 3 ) 2 1 . 8 × 10 5 0 . 178 mol 002 5.0 points Which is the strongest base? (Hint: Consider the relative strengths of the conjugate acids.) 1. ClO 3 2. ClO correct 3. Cl 4. ClO 4 5. ClO 2 Explanation: HCl, HClO 3 and HClO 4 are strong acids and so their bases are weak. Of the weak acids (HClO and HClO 2 ) HClO 2 is stronger, which means that the percentage of ClO 2 in solution is greater than the percentage of ClO . ClO is therefore the stronger base. 003 10.0 points For the reaction 2 NOCl(g) 2 NO(g) + Cl 2 (g) , if only [NOCl(g)] = 2.8 M initially, then at equilibrium [NO(g)]= 1.2 M. What is the equilibrium concentration of NOCl(g)? As- sume that only reactants exist initially. 1. 1.2 M 2. 0.8 M 3. 3.2 M 4. 1.6 M correct 5. 2.2 M Explanation: 004 5.0 points A: The standard reaction free energy at equi- librium is zero. B: A reaction stops when the equilibrium is reached. C: An equilibrium reaction is not affected by increasing the concentrations of products. Which of these statements is/are true? 1. Only A is true. 2. A, B, and C are true. 3. Only C is true.
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Version 067 – Exam 2 – Fakhreddine – (52395) 2 4. Only B and C are true. 5. Only A and C are true. 6. None is true. correct 7. Only B is true. 8. Only A and B are true. Explanation: A: false; The standard reaction free energy Δ G r is not 0 at equilibrium. The reaction free energy Δ G r , which is dependent upon the concentrations of the products and reactants, is 0 at equilibrium. B: false; Equilibrium is dynamic. At equi- librium, the concentrations of reactants and products will not change, but the reaction will continue to proceed in both directions. C: false; Equilibrium reactions are affected by the presence of both products and reac- tants. 005 10.0 points The pH of 0 . 1 M (C 3 H 7 NH 2 ) propylamine aqueous solution was measured to be 11 . 86. What is the value of p K b of propylamine? 1. 2.14 2. 11.86 3. 0.11 4. 3.25 correct 5. 0.96 6. 3.35 Explanation: M = 0 . 1 M pH = 11 . 86 Analyzing the reaction with molarities, C 3 H 7 NH 2 + H 2 O C 3 H 7 NH + 3 + OH 0 . 1 0 0 x x x 0 . 1 x x x pOH = 14 11 . 86 = 2 . 14 . [C 3 H 7 NH + 3 ] = [OH ] = 10 pOH = 10 2 . 14 = 0 . 00724436 mol / L The K b is K b = [C 3 H 7 NH + 3 ][OH ] [C 3 H 7 NH 2 ] = (0 . 00724436) 2 0 . 1 0 . 00724436 = 0 . 000565796 . and the p K b is p K b = log(0 . 000565796) = 3 . 24734 . 006 10.0 points Which solution has the highest pH?
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