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Quiz 6-solutions - Version 050 Quiz 6 fakhreddine(51650...

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Version 050 – Quiz 6 – fakhreddine – (51650) 1 This print-out should have 5 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 2.0points A solution is prepared by mixing 250 mL of 1.00 M CH 3 COOH with 500 mL of 1.00 M NaCH 3 COO. What is the pH of this solution? ( K a for CH 3 COOH = 1 . 8 × 10 - 5 ) 1. 4.58 2. 4.44 3. 5.36 4. 4.74 5. 5.05 correct Explanation: V CH 3 COOH = 250 mL [CH 3 COOH] = 1.00 M V NaCH 3 COO = 500 mL [NaCH 3 COO] = 1.00 M [CH 3 COOH] = 250 mmol 750 mL [CH 3 COO - ] = 500 mmol 750 mL pH = p K a + log [CH 3 COO - ] [CH 3 COOH] = - log(1 . 8 × 10 - 5 ) + log parenleftbigg 500 250 parenrightbigg = 5 . 04576 002 2.0points How many grams of NaF (of MW 42.0) would have to be added to 2.00 L of 0.100 M HF to yield a solution with a pH = 4.00? 1. 6.9 g 2. 300 g 3. 0.84 g 4. 60 g correct 5. 36 g Explanation: MW NaF = 42 g V = 2 L [HF] = 0.100 M pH = 4.00 K a = 7 . 2 × 10 - 4 pH = p K a + log parenleftbigg [F - ] [HF] parenrightbigg = p K a + log[F - ] - log[HF] log[F - ] = pH - p K a + log[HF] = 4 . 00 - [ - log(7 . 2 × 10 - 4 )] + log(0 . 1) = - 0 . 142668 [F - ] = 0 . 72 M = [NaF] 2 L soln × 0 . 72 mol NaF L soln × 42 g NaF 1 mol NaF = 60 . 48 g NaF 003 2.0points A buffer solution contains 0.25 M NaNO
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