exam 2 - Version 282 Exam 2 McCord (53580) 1 This print-out...

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Unformatted text preview: Version 282 Exam 2 McCord (53580) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. McCord 1pm for 0 = ax 2 + bx + c x =- b b 2- 4 ac 2 a 001 10.0 points The pH of 0 . 015 M HNO 2 (nitrous acid) aque- ous solution was measured to be 2 . 63. What is the value of p K a of nitrous acid? 1. 0.97 2. 3.59 3. 4.24 4. 3.36 correct 5. 2.63 6. 2.90 Explanation: M = 0 . 015 M pH = 2 . 63 Analyzing the reaction with molarities, HNO 2 + H 2 O H 3 O + + NO 2 . 015-- x- x x . 015- x- x x [H 3 O + ] = [NO 2 ] = 10 pH = 10 2 . 63 = 0 . 00234423 mol / L . The K a is K a = [H 3 O + ][NO 2 ] [HNO 2 ] = (0 . 00234423) 2 . 015- . 00234423 = 0 . 000434222 and the p K a is p K a =- log(0 . 000434222) = 3 . 36229 . 002 10.0 points If a solution containing dissolved AgNO 3 is added to a solution containing NaCl, then AgCl precipitates (forms a solid). The reac- tion can be written as NaCl(aq) + AgNO 3 (aq) AgCl(s) + NaNO 3 (aq) Remembering that NaCl(aq) and AgNO 3 (aq) are both strong electrolytes, the equilibrium constant for the reaction is best described by 1. K = [AgCl][Na + ][NO 3 ] [Na + ][Cl ][Ag + ][NO 3 ] 2. K = 1 [Cl ][Ag + ] correct 3. K = [NaNO 3 ] [NaCl][AgNO 3 ] 4. K = 1 [Na + ] 2 5. K = [AgCl] 2 [NaNO 3 ] [NaCl][AgNO 3 ] 6. K = [AgCl][Na + ] [Na + ][Cl ][Ag + ][NO 3 ] Explanation: 003 10.0 points Which of Acid X [HX] = 10 3 M K a = 1 . 32 10 6 Acid Y [HY] = 10 3 M K a = 3 . 47 10 3 is the stronger acid? 1. I would have to know if X or Y is a strong acid. 2. They have the same strength. 3. Acid X is stronger. 4. Acid Y is stronger. correct Version 282 Exam 2 McCord (53580) 2 Explanation: Acid Y has a larger K a value and is there- fore stronger. 004 10.0 points In a sample of pure water, only one of the fol- lowing statements is always true, at all con- ditions of temperature and pressure. Which one is always true? 1. [H 3 O + ] [OH ] = 1.0 2. pOH = 7.0 3. [OH ] = 1 . 10 7 M 4. [H 3 O + ] = [OH ] correct 5. pH = 7.0 Explanation: The value of K w = [H 3 O][OH ] changes with temperature. In a neutral solution [H 3 O + ] always equals [OH ]. Only when K w = 1 . 10 14 will pH and pOH equal 7.0 and [H 3 O + ] and [OH ] equal 1 . 10 7 M. 005 10.0 points A certain reaction has H equal to 11.57 kJ/mol. This reaction is normally run at room temperature (25 C). At what new tem- perature should the reaction be run so that K is twice its value at 25 C? 1. 65 C 2. 31 C 3. 77 C correct 4. 50 C 5. 135 C 6. 94 C Explanation: T 1 = 25 C +273 = 298 K K 2 = 2 K 1 H = 11570 J/mol ln K 2 K 1 = H R parenleftbigg 1 T 1- 1 T 2 parenrightbigg R H ln K 2 K 1 = 1 T 1- 1 T 2 T 2 = parenleftbigg 1 T 1- R H ln K 2 K 1 parenrightbigg 1 = parenleftbigg 1 298- 8 . 314 11570 ln2 parenrightbigg...
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exam 2 - Version 282 Exam 2 McCord (53580) 1 This print-out...

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