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Chemistry HW 6 - ekk86 hw06 Holcombe(53570 This print-out...

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ekk86 – hw06 – Holcombe – (53570) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. PLEASE NOTE: Due to the tolerances on numerical problems (your answer must be within ONE PERCENT of the correct answer unless otherwise noted), if a problem requires the quadratic formula to find the solution, DO NOT make any assumptions (i.e. USE THE QUADRATIC FORMULA). Also, be sure to enter excess significant figures to assure that you are not penalized for rounding. 001 10.0 points Solution A contains HCl and has an H + con- centration of 10 - 3 M. Solution B contains CH 3 COOH and has an H + concentration of 10 - 5 M. Which solution is more acidic and which solution has a greater pH? 1. Solution A is more acidic, but Solution B has a greater pH. correct 2. Both solutions have identical acidities and pH. 3. Solution B is more acidic, but Solution A has a greater pH. 4. Solution A is more acidic and has a greater pH. 5. Solution B is more acidic and has a greater pH. Explanation: [H + ] A = 10 - 3 M [H + ] B = 10 - 5 M pH = - log[H + ] The solutions have di ff erent concentrations of H + . Therefore, they will have di ff erent acidities and pH. The more concentrated will be more acidic and have a lower pH. 002 10.0 points The buildup of lactic acid in muscles causes pain during extreme physical exertion. The K a for lactic acid (C 2 H 5 OCOOH) is 8 . 4 × 10 - 4 . Calculate the pH of a 0 . 06 M solution of lactic acid. 1. 6.348 2. 2.174 correct 3. 2.149 4. 3.533 5. 11.851 6. 2.043 7. 2.297 8. 1.926 Explanation: K a = 8 . 4 × 10 - 4 [C 2 H 5 OCOOH] = 0 . 06 You MUST use the quadratic formula to solve this one. If you use the approximation, your answer will be more than 5% o ff . C 2 H 5 OCOOH + H 2 O Initial 0.06 Final 0 . 06 - x C 2 H 5 OCOO - + H 3 O + Initial - - Final x x K a = C 2 H 5 OCOO - H 3 O + [C 2 H 5 OCOOH] = x 2 0 . 06 - x = 8 . 4 × 10 - 4 x 2 + 8 . 4 × 10 - 4 x - 5 . 04 × 10 - 5 = 0 To solve the quadratic equation, x = - b ± b 2 - 4 a c 2 a Since b 2 - 4 a c = ( - 8 . 4 × 10 - 4 ) 2 - 4 (1) ( - 5 . 04 × 10 - 5 ) = 2 . 02 × 10 - 4 , then
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ekk86 – hw06 – Holcombe – (53570) 2 x = ( - 8 . 4 × 10 - 4 ) ± 2 . 02 × 10 - 4 2 (1) = (0 . 00669 , - 0 . 00753) , x = H 3 O + = 0 . 00669, so pH = - log H 3 O + = - log(0 . 00669) = 2 . 174 003 10.0 points The pH of 0 . 1 M HClO 2 (chlorous acid) aqueous solution was measured to be 1 . 2. What is the value of p K a for chlorous acid? 1. 1.20 2. 0.96 correct 3. 0.11 4. 1.40 5. 3.91 6. 2.57 Explanation: M = 0 . 1 M pH = 1 . 2 Analyzing the reaction with molarities, HClO 2 + H 2 O H 3 O + + ClO - 2 0 . 1 - 0 0 - x - x x 0 . 1 - x - x x [H 3 O + ] = [ClO - 2 ] = 10 - pH = 10 - 1 . 2 = 0 . 0630957 mol / L The K a is K a = [H 3 O + ][ClO - 2 ] [HClO 2 ] = (0 . 0630957) 2 0 . 1 - 0 . 0630957 = 0 . 107876 and the p K a is p K a = - log(0 . 107876) = 0 . 967077 .
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