Test 3 - Version 108 – Exam 3 – sutcliffe – (51060) 1...

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Unformatted text preview: Version 108 – Exam 3 – sutcliffe – (51060) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points What is the pH of a 0 . 15 M solution of methy- lammonium nitrate (CH 3 NH 3 NO 3 )? K b for methylamine (CH 3 NH 2 ) is 5 . × 10- 4 . 1. 6.35 2. 5.89 3. 2.06 4. 5.76 correct 5. 5.61 6. 11.9 7. 7.00 8. 5.52 Explanation: M CH 3 NH 3 NO 3 = 0 . 15 M K b = 5 . × 10- 4 It’s a salt of a weak base (BHX). This means you need a K a for the weak acid BH + . Use K a = K w K b and you’ll get the K a = 2 × 10- 11 . You CAN use the approximation for the equi- librium which means that [H + ] = radicalbig K a · C BH + = 1 . 73205 × 10- 6 M pH =- log(1 . 73205 × 10- 6 ) = 5 . 76144 002 10.0 points Consider mixing HNO 3 and NaNO 3 . This is NOT a buffer because 1. Na reacts with water to form OH- . 2. HNO 3 is not a weak acid. correct 3. NaNO 3 is not soluble. 4. NO- 3 reacts with water to form NO 2 (g). 5. NaNO 3 is basic. Explanation: 003 10.0 points What is [H 3 O + ] in a solution made by dissolving 0.200 mole of sodium acetate (NaCH 3 COO) in enough water to make one liter of solution? K a for CH 3 COOH is 1 . 80 × 10- 5 . 1. 4.98 2. 5 . 56 × 10- 11 3. 1 . 80 × 10- 6 4. 5.74 5. 9.25 6. 10.25 7. 1 . 05 × 10- 5 8. 9.02 9. 5 . 56 × 10- 10 10. 9 . 49 × 10- 10 correct Explanation: 004 10.0 points An indicator changes from yellow to blue go- ing from its acidic (nonionized) form to its basic (ionized) form. The indicator has a K a of 2 . 5 × 10- 8 . At pH 8.1, this indicator’s color would best be discribed as which color? 1. blue-green correct 2. yellow-green 3. blue 4. yellow 5. green Version 108 – Exam 3 – sutcliffe – (51060) 2 Explanation: The p K a is 7.60 so that is where you would see green (50/50 blend of yellow and blue). You would see all yellow for pH 6.60 and lower, and all blue for pH 8.60 and above. The pH 8.1 given is on the basic side of the middle so it would have a blue-green color. 005 10.0 points What is the pH of a solution containing 10- 9 M HClO 4 ? 1. 5.232 2. 8.768 3. 6.996 correct 4. 9.000 5. 5.000 Explanation: 006 10.0 points Consider the K sp data below and determine which two metal ions would be most easily separated using the chromate anion (CrO 2- 4 ). SrCrO 4 K sp = 2 . 6 × 10- 5 Ag 2 CrO 4 K sp = 9 . × 10- 12 PbCrO 4 K sp = 1 . 8 × 10- 14 BaCrO 4 K sp = 2 . 4 × 10- 10 1. Sr 2+ and Ag + 2. Sr 2+ and Ba 2+ 3. Ag + and Ba 2+ 4. Pb 2+ and Ba 2+ 5. Ag + and Pb 2+ 6. Sr 2+ and Pb 2+ correct Explanation: The two K sp which are farthest apart in values will reflect the two salts of CrO 2- 4 most dissimilar in solubility and most easy to sepa- rate....
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This note was uploaded on 02/06/2012 for the course CHEM 302 taught by Professor Mccord during the Fall '10 term at University of Texas.

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Test 3 - Version 108 – Exam 3 – sutcliffe – (51060) 1...

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