HW05-solutions

HW05-solutions - kamalska (mk23835) HW05 Quigley (104001)...

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kamalska (mk23835) – HW05 – Quigley – (104001) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 10.0 points In the series H±, HCl, HBr, HI, which is the strongest acid? 1. HI correct 2. 3. HBr 4. HCl Explanation: Going down the periodic table, the acidity oF the hydrogen halides increases. 002 10.0 points What is the pH oF a 0 . 3 M solution oF pyri- dinium chloride (C 5 H 5 NHCl)? K b For pyri- dine (C 5 H 5 N) is 1 . 5 × 10 9 . 1. 2.849 correct 2. 3.219 3. 9.327 4. 2.609 5. 2.979 6. 2.699 7. 5.176 8. 8.824 9. 7.000 10. 4.673 Explanation: M C 5 H 5 NHCl = 0 . 3 M K b = 1 . 5 × 10 9 It’s a salt oF a weak base (BHX). This means you need a K a For the weak acid BH + : K a = K w K b = 1 . 0 × 10 14 1 . 5 × 10 9 = 6 . 66667 × 10 6 You CAN use the approximation For the equilibrium which means that [H + ] = r K a · C BH + = R (6 . 66667 × 10 6 ) (0 . 3) = 0 . 00141421 M pH = - log(0 . 00141421) = 2 . 84949 003 10.0 points What is the pH oF an acid solution obtained by mixing 200 mL oF 0.0155 M H 2 SO 4 and 300 mL oF an 0.210 M HCl solution? Assume all oF the acids dissociate. 1. 1.9 2. 0.523 3. 0.460 4. 1.16 5. 1.2 6. 2.2 7. 0.859 correct Explanation: V H 2 SO 4 = 200 mL [H 2 SO 4 ] = 0.0155 M V HCl = 300 mL [HCl] = 0.210 M To determine the pH oF the fnal mixture, we need to determine how many moles H + is being contributed by each acid. ? mol H + = 0 . 200 L × 0 . 0155 mol H 2 SO 4 1 L × 2 mol H + 1 mol H 2 SO 4 = 0 . 0062 mol H +
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kamalska (mk23835) – HW05 – Quigley – (104001) 2 ? mol H + = 0 . 300 L × 0 . 210 mol HCl 1 L × 1 mol H + 1 mol HCl = 0 . 063 mol H + Add these two quantities together and divide by the total volume to get [H + ]. [H + ] = 0 . 063 mol + 0 . 0062 mol 0 . 2 L + 0 . 3 L = 0 . 138 M H + pH = - log[0 . 138] = 0 . 859 004 10.0 points Which of the acids I. HBrO 3 II. GaCl 3 III. HSO 4 IV. AlF 3 are classi±ed as Lewis acids but are not Bronsted-Lowry acids? 1. II and IV correct 2. I and IV 3. II and III 4. None of these 5. I and II Explanation: Lewis acids accept a share in an electron pair. A Bronsted-Lowry acid is a proton donor. I and III would be Bronsted acids. Therefore, only II and IV can be the correct answer. 005 10.0 points Calculate the hydrogen ion concentration for an aqueous solution that has a pH of 3.45. 1. 0.54 M 2. 2 . 8 × 10 11 M 3. 3 . 5 × 10 4 M correct 4. 1.22 M 5. 3 . 2 × 10 2 M Explanation: 006 10.0 points A substance is amphoteric if 1. it can act as either an acid or a base. correct 2. it is a strong acid. 3. it is a strong base. 4. it contains OH . 5.
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This note was uploaded on 11/04/2008 for the course CHEM 104 taught by Professor Quigley during the Spring '08 term at CUNY Hunter.

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HW05-solutions - kamalska (mk23835) HW05 Quigley (104001)...

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