CH302 Homework 5-solutions

CH302 Homework 5-solutions - Bodet (ngb299) Homework 5...

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Unformatted text preview: Bodet (ngb299) Homework 5 sutcliffe (51630) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. Please note: p just equals negative log of so we can have pH, pOH , pK a etc. This HW combined with HW 3 4 will cover us up to exam 2, so salts, neutralizations, buffers and indicators will be on Exam 3 (thats section 7.8, etc.) HW due the evening before the exam. 001 10.0 points How many grams of HNO 3 are needed to give a pH of 3 . 75 in an 11 . 5 L tank? The molar mass of HNO 3 is 63 . 1 g/mol. 1. 0.0112 g 2. 0.000178 g 3. 0.0000324 g 4. 1.36 g 5. 0.00204 g 6. 0.466 g 7. 5.49 g 8. 0.129 g correct 9. 2.19 g Explanation: pH = 3 . 75 V = 11 . 5 L MM = 63 . 1 g/mol [H + ] = 10- pH = 10- 3 . 75 = 0 . 000177828 Since HNO 3 is a strong acid, HNO 3 (aq) H + (aq) + NO- 3 (aq) . One HNO 3 one H + , so [H + ] = [HNO 3 ] = 0 . 000177828 M and (11 . 5 L) . 000177828 mol HNO 3 1 L = 0 . 00204502 mol HNO 3 . Thus (0 . 00204502 mol HNO 3 ) 63 . 1 g HNO 3 1 mol HNO 3 = 0 . 129041 g HNO 3 002 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. 3.91 2. 2.57 3. 0.11 4. 1.20 5. 0.96 correct 6. 1.40 Explanation: M = 0 . 1 M pH = 1 . 2 Analyzing the reaction with molarities, HClO 2 + H 2 O H 3 O + + ClO- 2 . 1-- x- x x . 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 . 1- . 0630957 = 0 . 107876 and the p K a is p K a =- log(0 . 107876) = 0 . 967077 . Bodet (ngb299) Homework 5 sutcliffe (51630) 2 003 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. zero. 2. < 5%. 3. None of these 4. the same. 5. > 5%. correct Explanation: 004 (part 1 of 3) 10.0 points A given weak acid HZ has a K a = 3 . 1 10- 6 ....
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CH302 Homework 5-solutions - Bodet (ngb299) Homework 5...

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