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Unformatted text preview: Version 017 – Exam 1 – Laude – (52455) 1 This printout should have 30 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 6.0 points Consider a reaction with Δ H rxn = 997 kJ · mol − 1 . Which of the following pairs of K values and temperatures is possible for this reaction? 1. K 1 = 15 . 7, T 1 = 138 K; K 2 = 1 . 57 × 10 38 , T 2 = 153 K correct 2. K 1 = 11 . 5, T 1 = 128 K; K 2 = 3 . 83, T 2 = 145 K 3. K 1 = 3 . 1, T 1 = 103 K; K 2 = 3 . 1, T 2 = 340 K 4. K 1 = 9 . 76, T 1 = 102 K; K 2 = 3 . 74 × 10 − 61 , T 2 = 116 K Explanation: For and endothermic reaction, K is directly related to T. For an exothermic reaction, K is inversely related to T. K is constant with temperature only for Δ H rxn = 0 kJ. 002 6.0 points Which combination of ΔG and K is possi ble? 1. Δ G = 29 . 4 J, K = 1 . 01 correct 2. Δ G = 77 . 6 kJ, K = 0 . 981 3. Δ G = 47 . 3 J, K = 3 . 02 × 10 13 4. Δ G = 54 . 7 kJ, K = 1 . 03 5. Δ G = 82 . 5 J, K = 3 . 15 × 10 − 10 Explanation: If ΔG is positive, K must be less than 1; if ΔG is negative, K must be greater than 1. Also, if ΔG is small (compared to RT), then K will be close to 1, and if ΔG is large, K will be much smaller or greater than 1. 003 6.0 points What would be pH of a 1 M solution of a very expensive weak base, unobtainamine, whose pK b is 3 . 4? 1. 12 . 3 correct 2. 14 3. 1 . 7 4. 7 5. Explanation: The equation [OH − ] = (K b · C b ) . 5 can be algebraically rearranged to pOH = 0 . 5(pK b + pC b ) = 0 . 5(3 . 4 + 0) = 1 . 7 pH = 14 pOH = 12 . 3 004 6.0 points Consider the chemical equation below: N 2 O 4 (g) + 4H 2 (g) ←→ 4H 2 O(g) + N 2 (g) What would K eq be for this reaction? 1. K eq = [H 2 O] 4 [N 2 ] [N 2 O 4 ][H 2 ] 4 correct 2. K eq = [N 2 O 4 ][H 2 ] 4 [H 2 O] 4 3. K eq = [H 2 O] 4 [N 2 O 4 ][H 2 ] 4 4. K eq = [N 2 O 4 ][H 2 ] 4 [H 2 O] 4 [N 2 ] Explanation: For a reaction of the form: aA(s) + bB(aq) ↔ cC( ℓ ) + dD(g) The equilibrium constant will take the form: K eq = [D] d [B] b Version 017 – Exam 1 – Laude – (52455) 2 Only species that have a changeable con centration, gases and aqueous species, have activities that change. Solids and liquids have activities which are constants, so they don’t appear in mass action expressions. 005 6.0 points If ethylene glycol has a pure vapor pressure of 28 torr and acetaldehyde has a pure vapor pressure of 756 torr A mixture of 6 moles of ethylene glycol and 1 moles of acetaldehyde would have what total vapor pressure? 1. 672 torr 2. 24 torr 3. 112 torr 4. 108 torr 5. 132 torr correct Explanation: P total = χ a · P a + χ b · P b + ... P total = 0 . 857143 · 28 torr + 0 . 142857 · 756 torr P total = 132 torr 006 6.0 points For a solution with a pH of 1, what would be the value of [H + ] , [OH − ] and pOH, respec tively?...
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 Spring '07
 Holcombe
 Reaction, Exothermic, Phase transition, Laude

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