HW7_Solutions

# HW7_Solutions - estrada (gse77) – Homework 7 –...

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Unformatted text preview: estrada (gse77) – Homework 7 – Sutcliffe – (52410) 1 This print-out should have 22 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. NOTE: a p in front of a K a or K b value means to get the actual value of K a or K b , you must use the 10 x button on your calculator, in the same way you find [H + ] from pH. SEC- OND NOTE: Hydrolyse means ’react with water’ 001 10.0 points A solution of 0.2 M boric acid is prepared as an eye wash. What is the approximate pH of this solution? For boric acid K a = 7 . 2 × 10- 10 . 1. pH = 3 2. pH = 4 3. pH = 5 correct 4. pH = 6 5. pH = 7 Explanation: 002 10.0 points The pH of 0 . 015 M HNO 2 (nitrous acid) aque- ous solution was measured to be 2 . 63. What is the value of p K a of nitrous acid? 1. 0.97 2. 3.36 correct 3. 3.59 4. 4.24 5. 2.63 6. 2.90 Explanation: M = 0 . 015 M pH = 2 . 63 Analyzing the reaction with molarities, HNO 2 + H 2 O ⇀ ↽ H 3 O + + NO- 2 . 015-- x- x x . 015- x- x x [H 3 O + ] = [NO- 2 ] = 10- pH = 10- 2 . 63 = 0 . 00234423 mol / L . The K a is K a = [H 3 O + ][NO- 2 ] [HNO 2 ] = (0 . 00234423) 2 . 015- . 00234423 = 0 . 000434222 and the p K a is p K a =- log(0 . 000434222) = 3 . 36229 . 003 10.0 points A 7 . 2 × 10- 3 M solution of acetic acid is 5.0% dissociated. In a 7 . 2 × 10- 4 M solution, the percent dissociation would be 1. < 5%. 2. zero. 3. > 5%. correct 4. the same. 5. None of these Explanation: 004 10.0 points The amide ion NH- 2 can react with water to form ammonia (NH 3 ) and a hydroxide ion. Which is true about this reaction? 1. Water and ammonia are a conjugate acid- base pair. 2. The hydroxide ion is the conjugate base of the amide ion. 3. NH 3 is the conjugate acid of NH- 2 . cor- rect estrada (gse77) – Homework 7 – Sutcliffe – (52410) 2 4. Water and the amide ion are a conjugate acid-base pair. 5. The amide ion is acting as an acid....
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## This note was uploaded on 03/03/2012 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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HW7_Solutions - estrada (gse77) – Homework 7 –...

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