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# Answer Key_Exam 3 - Version 044 Exam 3 rowland(53210 This...

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Version 044 – Exam 3 – rowland – (53210) 1 This print-out should have 22 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Choose the pair of concentrations that cannot be in a given aqueous solution at 25 C. 1. [H 3 O + ] = 10 14 M : [OH ] = 1 M 2. Any of these could exist. correct 3. [H 3 O + ] = 10 3 M : [OH ] = 10 11 M 4. [H 3 O + ] = 10 M : [OH ] = 10 15 M 5. [H 3 O + ] = 10 7 M : [OH ] = 10 7 M Explanation: All of these concentrations are possible. Re- call that pH can exist beyond the 1-14 range, but at 25 C, pH + pOH = 14 always applies. 002 10.0 points According to the Arrhenius Theory, an acid produces ? and a base produces ? . 1. hydroxide ions (OH ); hydrogen ions (H + ) 2. protons; electrons 3. hydrogen ions (H + ); hydroxide ions (OH ) correct 4. electrons; protons Explanation: The Arrhenius theory of acids and bases defines acids as substances which ionize in water to produce hydrogen ions and bases as substances which in water give off hydroxide ions. 003 10.0 points What is [OH ] in a 0.0050 M HCl solution? 1. 5 . 0 × 10 3 M 2. 6 . 6 × 10 5 M 3. 1 . 0 M 4. 1 . 0 × 10 7 M 5. 2 . 0 × 10 12 M correct Explanation: [OH ] = 0 . 0050 M Since HCl is a strong acid, it completely dissociates and H + is 0 . 0050 M. HCl H + + Cl K w = [H + ][OH ] = 1 × 10 14 [OH ] = K w [H + ] = 1 × 10 14 0 . 0050 = 2 × 10 12 M 004 10.0 points Carbonic acid (H 2 CO 3 ) is a diprotic acid with K a1 = 4 . 2 × 10 7 and K a2 = 4 . 8 × 10 11 . The ion product for water is K w = 1 . 0 × 10 14 . What is the [H 3 O + ] concentration in a saturated carbonic acid solution that is 0.037 molar? 1. 7 . 4 × 10 2 M 2. 4 . 2 × 10 7 M 3. 6 . 5 × 10 4 M 4. 1 . 2 × 10 4 M correct 5. 3 . 7 × 10 20 M Explanation: 005 10.0 points Calculate the concentration of H 2 SO 3 present in 0 . 123 M Na 2 SO 3 (aq). 1. None of these 2. 4 . 16667 × 10 8 M

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Version 044 – Exam 3 – rowland – (53210) 2 3. 8 . 33333 × 10 8 M 4. 3 . 33333 × 10 13 M 5. 1 . 33333 × 10 12 M 6. 6 . 66667 × 10 13 M correct Explanation: K a1 = 0 . 015 K a2 = 1 . 2 × 10 7 K w = 1 × 10 14 The equilibrium reactions of interest are now the base forms of the carbonic acid equi- libria, so K b values should be calculated for the following changes: SO 2 3 (aq) + H 2 O( ) OH (aq) + HSO 3 (aq) K b1 = K w K a2 = 1 × 10 14 1 . 2 × 10 7 = 8 . 33333 × 10 8 . HSO 3 (aq) + H 2 O( ) OH (aq) + H 2 SO 3 (aq) K b2 = K w K a1 = 1 × 10 14 0 . 015 = 6 . 66667 × 10 13 . Because the second hydrolysis constant is much smaller than the first, we can assume that the first step dominates: SO 2 3 + H 2 O OH + HSO 3 0 . 123 0 0 x + x + x 0 . 123 x x x Assuming that x 0 . 123 M, K b1 = [OH ][HSO 3 ] [SO 2 3 ] 8 . 33333 × 10 8 = x 2 0 . 123 x x 2 0 . 123 x 2 = (0 . 123) (8 . 33333 × 10 8 ) x = 0 . 000101242 M .
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Answer Key_Exam 3 - Version 044 Exam 3 rowland(53210 This...

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