HW4solutions - Niu (qn269) – Homework 4 – flowers –...

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Unformatted text preview: Niu (qn269) – Homework 4 – flowers – (53885) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. HW 4. Due Tuesday 07 October 2008 11 PM. 001 (part 1 of 2) 10.0 points A 0 . 77 g sample of sodium hydrogen sulfite (NaHSO 3 ) is dissolved in 64 mL of water. What is the pH of the solution? p K a 1 = 1 . 81 and p K a 2 = 6 . 91. 1. 8.14 2. None of these 3. 5.64 4. 7.0 5. 4.36 correct 6. 6.45 Explanation: p K a 1 = 1 . 81 p K a 2 = 6 . 91. This is a salt of a polyprotic acid which will dissociate in solution. The cation is an extremely weak acid and will not affect the equilibrium. HSO − 3 can act as both an acid and a base. Both actions need to be consid- ered simultaneously: HSO − 3 + H 2 O ⇀ ↽ H 3 O + + SO 2 − 3 HSO − 3 + H 2 O ⇀ ↽ H 2 SO 3 + OH − and The extent to which these processes occur will be determined by the relative values of p K a 1 and p K a 2 . To find pH we use the following: pH = 1 2 (p K a 1 + p K a 2 ) = 1 2 (1 . 81 + 6 . 91) = 4 . 36 . Note that the pH is independent of the con- centration of the anion as long as it is not extremely dilute. At extremely low concentrations of HSO − 3 , however, the approximations upon which the use of this equation are based are no longer valid. 002 (part 2 of 2) 10.0 points What is the pH of a solution resulting from the dissolution of 60 mg of AgNO 3 in 15 mL of water? 1. 4.36 2. 7.0 correct 3. 5.64 4. None of these 5. 6.45 6. 8.14 Explanation: Neither the silver ion nor the nitrate ion is acidic or basic in aqueous solution, so the pH is of the neutral water: 7. 003 10.0 points Rank the following 1.0 M solutions NaCN, H 2 S, RbOH, CaCl 2 , HI in order of DECREASING pH. 1. RbOH, CaCl 2 , HI, NaCN, H 2 S 2. CaCl 2 , NaCN, H 2 S, HI, RbOH 3. NaCN, CaCl 2 , RbOH, HI, H 2 S 4. RbOH, NaCN, CaCl 2 , H 2 S, HI correct 5. H 2 S, HI, NaCN, RbOH, CaCl 2 Explanation: 004 10.0 points What is the [H 3 O + ] in a 0 . 050 M solution of Ba(OH) 2 ? 1. 5 . × 10 − 2 M Niu (qn269) – Homework 4 – flowers – (53885) 2 2. 2 . × 10 − 5 M 3. 2 . × 10 − 13 M 4. 1 . × 10 − 5 M 5. 1 . × 10 − 13 M correct 6. 5 . × 10 − 10 M Explanation: [Ba(OH) 2 ] = 0 . 050 M Ba(OH) 2 is a strong base and completely ionizes in aqueous solution. Ba(OH) 2 → Ba 2+ + 2 OH − ini, M 0.05 Δ, M − . 05 +0 . 05 +2(0 . 05) fin, M . 05 . 10 [OH − ] = 0 . 10 = 1 . × 10 − 1 M K w = [H 3 O + ][OH − ] = 1 × 10 − 14 [H 3 O + ] = K w [OH − ] = 1 . × 10 − 14 1 . × 10 − 1 = 1 . × 10 − 13 M 005 10.0 points Which acid has the strongest conjugate base? 1. HBr 2. HF correct 3. HCl 4. HI Explanation: HBr, HCl and HI are strong acids which completely ionize in water so their conjugate base is a poor proton acceptor. HF is a weak acid which is only partially ionized in water....
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This note was uploaded on 02/04/2009 for the course CH 53885 taught by Professor Flowers during the Fall '08 term at University of Texas.

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HW4solutions - Niu (qn269) – Homework 4 – flowers –...

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