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Exam 3-solutions

# Exam 3-solutions - Version 020 Exam 3 fakhreddine(51615...

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Version 020 – Exam 3 – fakhreddine – (51615) 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 10.0points A 130 mL portion of 0 . 4 M acetic acid is be- ing titrated with 0 . 4 M NaOH solution. What is the pH of the solution after 110 mL of the NaOH solution has been added? The ioniza- tion constant of acetic acid is 1 . 8 × 10 - 5 . 1. pH = 5 . 83 2. pH = 5 . 49 correct 3. pH = 3 . 99 4. pH = 6 . 22 5. pH = 4 . 44 Explanation: V CH 3 COOH = 130 mL [CH 3 COOH] = 0 . 4 M V NaOH = 110 mL [NaOH] = 0 . 4 M K a = 1 . 8 × 10 - 5 For CH 3 COOH, (0 . 4 M)(0 . 13 L) = 0 . 052 mol For NaOH, (0 . 4 M)(0 . 11 L) = 0 . 044 mol CH 3 COOH + NaOH NaCH 3 COO + H 2 O 0 . 052 mol 0 . 044 mol 0 mol 0 . 044 mol 0 . 044 mol +0 . 044 mol 0 . 008 mol 0 mol 0 . 044 mol CH 3 COOH CH 3 COO - + H + 0 . 008 mol 0 . 044 mol 0 . 24 L 0 . 24 L 0 . 0333333 M 0 . 183333 M x Thus K a = bracketleftbig CH 3 COO - bracketrightbigbracketleftbig H + bracketrightbig [CH 3 COOH] 1 . 8 × 10 - 5 = 0 . 183333 x 0 . 0333333 x = bracketleftbig H + bracketrightbig = K a [CH 3 COOH] [CH 3 COO - ] = ( 1 . 8 × 10 - 5 ) (0 . 0333333) 0 . 183333 = 3 . 27273 × 10 - 6 Thus pH = log bracketleftbig H + bracketrightbig = 5 . 48509 002 10.0points Which of the following acid-base indicators should be used for the titration of KOH with HBr? 1. Methyl red, color change red/yellow at 4 . 4 < pH < 6 . 2 2. Phenolphthalein, color change colorless/red-violet 8 . 0 < pH < 10 . 0 3. Any of these is suitable. correct 4. Neutral red, color change red/yellow 6 . 8 < pH < 8 . 0 5. None of these is suitable. Explanation: A strong acid - strong base titration goes from a very high/low pH to very low/high pH with endpoint at pH 7. The pH change experienced by this titration can cause any of these indicators to change color. 003 10.0points What is the molar solubility of Ag 2 S? The K sp is 6 . 3 × 10 - 51 . 1. 2 . 82 × 10 - 13 2. 6 . 37 × 10 - 15 3. 5 . 8 × 10 - 18 4. 1 . 16 × 10 - 17 correct 5. 7 . 94 × 10 - 26 Explanation: 004 10.0points Which of the following indicators would be most suitable for the titration of 0.10 M lactic

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Version 020 – Exam 3 – fakhreddine – (51615) 2 acid with 0.10 M KOH(aq)? For lactic acid, p K a = 3.08. 1. phenol red (p K In = 7.9) correct 2. bromophenol blue (p K In = 3.9) 3. methyl orange (p K In = 3.4) 4. alizarin yellow (p K In = 11.2) 5. thymol blue (p K In = 1.7) Explanation: 005 10.0points When ammonium chloride is added to NH 3 (aq), 1. the pH of the solution does not change. 2. the pH of the solution decreases. correct 3. the K b increases. 4. the equilibrium concentration of NH 3 (aq) decreases. 5. the pH of the solution increases. Explanation: 006 10.0points Ka = 7.2x10 - 4 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. 60 g correct 2. 300 g 3. 36 g 4. 6.9 g 5. 0.84 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 007 10.0points What is the concentration of hydrogen ions in a solution containing 0 . 468 M HOCN (cyanic acid) and 0 . 387 M (NaOCN) (sodium cyanate)? The ionization constant of cyanic acid is 3 . 5 × 10 - 4 .
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