Exam 3-solutions

# Exam 3-solutions - Version 028 – Exam 3 – mccord –...

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Unformatted text preview: Version 028 – Exam 3 – mccord – (51625) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. mccord-ch302 2pm class only - 51625 K w = 1 . × 10 − 14 1 F = 96485 C/mol e − Standand Potentials at 25 ◦ C Cl 2 + 2 e −-→ 2 Cl − +1.36 V Ag + + e −-→ Ag +0.80 V Hg 2+ 2 + 2 e −-→ 2 Hg +0.79 V Cu 2+ + 2 e −-→ Cu +0.34 V AgCl + e −-→ Ag + Cl − +0.22 V 2 H + + 2 e −-→ H 2 0.000 V Ti 3+ + e −-→ Ti 2+-0.37 V Zn 2+ + 2 e −-→ Zn-0.76 V Mn 2+ + 2 e −-→ Mn-1.18 V Mn 3+ + e −-→ Mn 2+-1.51 V Ti 2+ + 2e −-→ Ti-1.63 V Al 3+ + 3 e −-→ Al-1.66 V 001 4.0 points If a solution of HOCl is titrated by a strong base such as NaOH, what is the pH at the half equivalence point? The ionization constant is 3 . 5 × 10 − 8 for HOCl. 1. pH = 5.5 2. pH = 6.5 3. pH = 3.5 4. pH = 7.5 correct 5. pH = 4.5 Explanation: K a = 3 . 5 × 10 − 8 At half the equivalence point, this solution is a buffer in which [HOCl] = [OCl − ]. pH = p K a + log parenleftbigg [OCl − ] [HOCl] parenrightbigg = p K a =- log(3 . 5 × 10 − 8 ) = 7 . 45593 002 4.0 points Consider two hypothetical metals, X and Y, which can exist as metal ions X 2+ and Y 3+ , respectively. For these two metals, the stan- dard reduction potentials are X 2+ + 2 e − → X (0 . 182 V) Y 3+ + 3 e − → Y (0 . 205 V) What is the value of Δ G at 25 ◦ C for the reaction 3 X(s) + 2 Y 3+ (aq) → 3 X 2+ (aq) + 2 Y(s) ? 1. 75.8375 2. -46.313 3. 89.7892 4. -13.315 5. 155.727 6. -74.1007 7. 1.15782 8. 161.922 9. 100.731 10. 43.9973 Correct answer:- 13 . 315 kJ. Explanation: 003 4.0 points What is the pH of a solution containing 100 mL of 0.5 M NH 3 and 200 mL of 0.1 M HCl? The p K b of ammonia is 4.75. 1. 9.65 2. 9.95 3. 8.72 4. 9.43 correct Version 028 – Exam 3 – mccord – (51625) 2 5. 9.15 Explanation: 004 4.0 points What is the pH at the stoichiometric point for the titration of 0.26 M CH 3 NH 2 (aq) with 0.26 M HClO 4 (aq)? For CH 3 NH 2 , K b = 3 . 6 × 10 − 4 . 1. 2.16 2. 5.57 3. 7.00 4. 5.72 correct 5. 2.01 Explanation: 005 4.0 points For an electrochemical reduction process, electrons are 1. added to solution species at the surface of the anode. 2. taken from solution species at the surface of the cathode. 3. added to solution species at the surface of the cathode. correct Explanation: 006 4.0 points What would be the pH if 0.030 moles of solid NaOH were added to 200 mL of 0.200 M acetic acid solution? Assume the volume of the solution does not change when the solid is added. The ionization constant of acetic acid is 1 . 8 × 10 − 5 . 1. None of these 2. pH = 4.27 3. pH = 5.35 4. pH = 4.87 5. pH = 5.22 correct Explanation: [CH 3 COOH] = 0.2 M V CH 3 COOH = 200 mL K a = 1 . 8 × 10 − 5 Initial condition (ini): n NaOH = 0 . 03 mol n CH 3 COOH = 0 . 200 × . 200 = 0 . 04 mol NaOH + CH 3 COOH → Na + + CH 3 COO − + H 2 O . 03 . 04- . 03- . 03 . 03 . 03 .....
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## This note was uploaded on 07/12/2011 for the course GOV 365L taught by Professor Liu during the Spring '08 term at University of Texas.

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Exam 3-solutions - Version 028 – Exam 3 – mccord –...

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