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Exam 1, Spring 2011

# Exam 1, Spring 2011 - Version 406 – Exam 1 – laude...

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Unformatted text preview: Version 406 – Exam 1 – laude – (51635) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. Constants R = 8 . 314 J · K − 1 · mol − 1 Definitions Q ≡ [C] c [D] d [A] a [B] b G ≡ H- TS K w ≡ [H + ][OH − ] pH ≡ - log [H + ] pOH ≡ - log [OH − ] Equations Δ G = Δ H- T Δ S ln parenleftbigg P 2 P 1 parenrightbigg = Δ H ◦ vap R parenleftbigg 1 T 1- 1 T 2 parenrightbigg Π = MRT Δ T b = iK b m Δ T f = iK f m P i = P ◦ i χ i P = summationdisplay i P i q = mc Δ T q = m Δ H Δ G ◦ =- RT ln K K = e − Δ G ◦ RT ln K 2 K 1 = Δ H ◦ R parenleftbigg 1 T 1- 1 T 2 parenrightbigg Q = K [at equil.] [H + ] = ( K a C a ) 1 / 2 [OH − ] = ( K b C b ) 1 / 2 [H + ] = K a ( C a /C b ) pH = p K a + log( C b /C a ) [OH − ] = K b ( C b /C a ) pOH = p K b + log( C a /C b ) 001 6.0 points Which of the following equations is untrue for a solution with a pH of 3? 1. [OH − ] = K w 10 − 3 2. [H + ] = 10 − 3 3. pOH =- log pK w 10 − 3 correct 4. all of these equations are true 5. pOH =- log K w 10 − 3 Explanation: pOH =- log[OH − ] K w = [OH − ][H + ] pOH =- log K w [H + ] =- log K w 10 − 3 It should be clear that K w , not pK w belongs in the correct expression for pOH. 002 6.0 points Rank the following liquids by their misci- bility in heptane (C 7 H 16 ), from most miscible to least: NH 3 , CH 3 OH, CH 3 CH 2 F, CCl 4 . 1. CH 3 CH 2 F > CH 3 OH > CCl 4 > NH 3 2. NH 3 > CH 3 OH > CH 3 CH 2 F > CCl 4 3. CCl 4 > CH 3 CH 2 F > CH 3 OH > NH 3 correct 4. CH 3 CH 2 F > CCl 4 > CH 3 OH > NH 3 5. CCl 4 > CH 3 CH 2 F > NH 3 > CH 3 OH Explanation: ”Like dissolves like.” Heptane is hydrocar- bon, and thus completely non-polar. Car- bon tetrachloride is most soluble since even though the C-Cl bond are polar, there is no net dipole in the molecule, followed by fluo- roethane, which is polar. Methanol is small and very polar (exhibiting hydrogen bond- ing), and ammonia is the most polar of the Version 406 – Exam 1 – laude – (51635) 2 liquids, and thus least soluble. 003 6.0 points Which of the following is a possible combina- tion of values for ΔH lattice , ΔH hydration and ΔH solution , respectively, for a salt whose dis- solution is endothermic. 1. +550 ,- 480 , and + 1030 kJ · mol − 1 , 2.- 900 ,- 900 , and- 1800 kJ · mol − 1 , 3.- 450 , +400 , and- 50 kJ · mol − 1 , 4. +640 ,- 620 , and + 20 kJ · mol − 1 , cor- rect Explanation: Δ H solution = Δ H lattice + Δ H hydration- the problem stipulates that Δ H solution be positive (endothermic) and this limits the values the other two terms can have. 004 6.0 points A container holds a mixture of acetone and water at 40 ◦ C. If χ acet , the mole fraction of acetone, is 0.1, what is the total vapor pressure? (The equilibrium vapor pressures for pure acetone and pure water at 40 ◦ C are roughly 400 torr and 50 torr respectively.) 1. 40 torr 2. 85 torr correct...
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Exam 1, Spring 2011 - Version 406 – Exam 1 – laude...

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