Homework 6-solutions

1 This print-out should have 14 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 1.0points Which is NOT a conjugate acid-base pair? 1. HCl : Cl - 2. H 2 SO 4 : SO 2 - 4 correct 3. H 3 SO + 4 : H 2 SO 4 4. H 2 O : OH - 5. H 2 : H - Explanation: Except for H 2 SO 4 and SO 2 - 4 , the members of all of the pairs differ by one proton. 002 1.0points For a given weak acid HA the value of K a 1. cannot be less than 10 - 7 . 2. changes if we add a strong acid to a solution of the weak acid. 3. does not change with temperature. 4. changes if we add a strong base to a solution of the weak acid. 5. None of these correct 6. cannot be greater than 10 - 7 . 7. changes with the pH of the solution. Explanation: 003 1.0points Consider five generic acids (HA, HB, HC, HD, and HE) that have the following ionization constants. Ionization Acid Constant K a value HA 4 . 6 × 10 - 3 HB 1 . 3 × 10 - 3 HC 7 . 6 × 10 - 4 HD 6 . 3 × 10 - 3 HE 9 . 3 × 10 - 5 Which of the following anions will be the WEAKEST base? 1. A - 2. B - 3. C - 4. E - 5. D - correct Explanation: The weakest base anion will pair up with the strongest acid in the list which is HD. 004 1.0points Given the following p K a values, rank the conjugate bases in order of increasing strength. I) HF p K a = 3 . 45 II) HCN p K a = 9 . 31 III) HNO 2 p K a = 3 . 37 IV) HCOOH p K a = 3 . 75 V) H 2 CO 3 p K a = 6 . 37 1. II, V, IV, I, III 2. V, III, I, II, IV 3. II, IV, V, III, I 4. III, I, IV, V, II correct 5. I, II, III, IV, V Explanation:

2 005 1.0points Normal rain has a pH of about 5.5 whereas the most acidic rain measured in Los Angeles had a pH of 1.5. How much more concentrated in H + is the acid rain than the normal rain? Correct answer: 10000. Explanation: 006 1.0points Consider the following data phenol (C 6 H 5 OH) pK a = 9.99 chlorous acid (HClO 2 ) pK a = 1.96 pyridine (C 5 H 5 N) pK b = 8.77 aniline (C 6 H 5 NH 2 ) pK b = 9.42 Which of the following correctly lists bases arranged from weakest to strongest? 1. C 6 H 5 OH , C 6 H 5 NH 2 , C 5 H 5 N , HClO 2 2. C 6 H 5 O - , ClO - 2 , C 5 H 5 N , C 6 H 5 NH 2 3. ClO - 2 , C 6 H 5 NH 2 , C 5 H 5 N , C 6 H 5 O - cor- rect 4. C 6 H 5 O - , C 6 H 5 NH 2 , C 5 H 5 N , ClO - 2 5. HClO 2 , C 6 H 5 NH 2 , C 5 H 5 N , C 6 H 5 OH 6. C 6 H 5 OH , HClO 2 , C 5 H 5 N , C 6 H 5 NH 2 Explanation: The value of pK b is inversely proportional to the basicity (strength) of the base. By ranking from the largest (12.04) to the small- est (4.01) pK b , one is ranking from the least to most basic. pK b + pK a = pK w = 14 at room temperature. 007 1.0points Like all equilibrium constants, K w varies somewhat with temperature. Given that K w is 4 . 95 × 10 - 13 at some temperature, what is the pH of a neutral aqueous solution at that temperature? 1. 6.43 2. 6.22 3. 6.34 4. 6.06 5. 6.15 correct Explanation: 008 1.0points What is the pOH of a 0 . 09 M solution of Ba(OH) 2 ? Assume complete dissociation of the compound in water. 1. 12 . 95 2. 13 . 26 3. 0 . 74 correct 4. 1 . 05 Explanation: [Ba(OH) 2 ] = 0 . 09 M Ba(OH) 2 → Ba 2+ + 2 OH - 0 . 09 - - 0 . 09 - 0 . 09 0 . 09 2(0 . 09) 0 0 . 09 0 . 18 pOH = - log[OH - ] = - log(0 . 18) = 0 . 74 009 1.0points The pH of a solution of Ba(OH) 2 is 9.40. What is the molarity of the Ba(OH) 2 solu- tion? 1. 5 . 0 × 10 - 5 M 2. 2 . 5 × 10 - 5 M 3. 1 . 8 × 10 - 5 M 4. 8 . 3 × 10 - 4 M 5. 1 . 3 × 10 - 5 M correct Explanation: 010 1.0points

4 014 1.0points