HW 5 - – Homework #5 – holcombe 1 This print-out should...

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Unformatted text preview: – Homework #5 – holcombe 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A solution of CO 2 dissolved in water is 1. neutral. 2. acidic. correct 3. basic. 4. precipitated. Explanation: 002 10.0 points What is the conjugate base of H 2 PO- 4 ? 1. H 2 PO- 4 2. OH- 3. H 3 PO 4 4. PO 3- 4 5. HPO 2- 4 correct Explanation: 003 10.0 points A change in pH of 1.0 unit indicates that 1. the molarity of H + changes by one order of magnitude. correct 2. the acid or base content decreases or in- creases by 1.0 M. 3. the salt content decreases or increases by 1.0 M. 4. the acid or base content decreases or in- creases by 0.1 M. 5. the volume of H + in a solution changes by an order of magnitude. Explanation: If the pH of a solution is 1.0, that means that the H + concentration is 0.1 M. If the pH is 2.0, the H + concentration is 0.01 M. The difference is one order of magnitude. 004 10.0 points What would be the pH of a solution of hypo- bromous acid (HOBr) prepared by dissolving 9.7 grams of the acid in 20 mL of pure wa- ter (H 2 O)? The Ka of hypobromous acid is 2 × 10- 9 1. 4 correct 2. 6 3. 13 4. 1 5. 10 Explanation: 9 . 7 g HOBr × 1 mol 97 g = 0 . 1 mol HOBr . 1 mol HOBr . 02 L H 2 O = 5 M HOBr [H + ] = (K a · C a ) 1 / 2 = (2 × 10- 9 · 5) 1 / 2 = (10- 8 ) 1 / 2 = 10- 4 pH =- log[H + ] =- log(10- 4 ) = 4 005 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. 0.435 moles 2. 5.69 moles 3. 0.0101 moles 4. 0.178 moles correct 5. 9.88 moles Explanation: – Homework #5 – holcombe – 2 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 ≈ . 178 mol 006 10.0 points The pH of 0 . 1 M (C...
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This note was uploaded on 11/12/2010 for the course CH 302 taught by Professor Holcombe during the Fall '07 term at University of Texas.

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HW 5 - – Homework #5 – holcombe 1 This print-out should...

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