Chem 162-2011 exam I + solutions

# Chem 162-2011 exam I + solutions - CHEM 162-2011 EXAM I...

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1 Chem 162-2011 Exam I th edition Chapter 13B – Kinetics The reaction 2A + B → C has the following mechanism. A + B   D fast equilibrium D + B → E slow step E + A → C + B fast step Which rate law is consistent with the above mechanism? A) Rate = k [A] 2 [B] B ) Rate = k [A][B] 2 C) Rate = k [A][B] D) Rate = k [A] 2 [B] 2 E) Rate = k [A ] 2 The rate is based on the slow step. Rate = [D][B] D is a reactive intermediate and therefore is not in the experimentally determined stoichiometric equation. Therefore, replace [D] Rate forward = Rate reverse k 1 = rate forward in the fast equilibrium step k -1 = rate reverse in the fast equilibrium step Rate forward = k 1 x [A][B] Rate reverse = k -1 x [D] k 1 [A][B] = k -1 [D] [D] = (k 1 /k -1 )[A][B] Rate = ((k 1 /k -1 )[A][B]) x [B] = k’[A][B] 2 B CHEM 162-2011 EXAM I + SOLUTIONS

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2 Chem 162-2011 Exam I th edition Chapter 12 – Properties of Solutions Vapor pressure of solutions calculations The vapor pressure of pure benzene (C 6 H 6 ) and toluene (C 7 H 8 ) at 25°C are 95.1 and 28.4 Torr, respectively. A solution is prepared with a mole fraction of toluene of 0.750. Determine the mole fraction of toluene in the gas phase. Assume the solution to be ideal. A) 0.750 B) 0.250 C) 0.213 D ) 0.473 E) 0.0280 P soln = P X solventA P o solventA + P X solventB P o solventB X benzene = 1 - 0.750 = 0.250 X toluene = 0.750 P o benzene = 95.1 Torr P o toluene = 28.4 Torr i = 1 for benzene and toluene P bsoln = (0.250 x 95.1) = 23.78 mm P tsoln = (0.750 x 28.4) = 21.30 mm The moles are directly related to the vapor pressure, according to Dalton’s Law. Therefore: X tolueneinvapor = 21.30/(21.30 + 23.78) = 0.472 D
Chem 162-2011 Exam I th edition Chapter 12 – Properties of Solutions Vapor pressure of solutions concepts (Henry’s Law, Raoult’s Law) Which one of the following statements is true ? X. A negative deviation from Raoult’s law means that the observed pressure will be lower than that predicted by Raoult’s law. Y. Negative deviations from Raoult’s law are expected when ΔH sol’n is positive (endothermic). Z. Strong interactions between solute and solvent result in negative deviations from Raoult’s law. A. X only B. X and Y only C . X and Z only D. Y and Z only E. Z only X. True: An example of a negative deviaion from Raoult’s law would be a mixture of acetone and chloroform. An acetone-acetone mixture is held together by dipole-dipole interaction. A chloroform-chloroform mixture is held together by dipole-dipole interaction. An acetone-chloroform mixture is held together by hydrogen bonding, stronger interaction than dipole-dipole interaction. This stronger interaction results in lower vapor pressure than predicted by Raoult’s law. Y.

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## This document was uploaded on 11/02/2011 for the course GEN CHEM 162 at Rutgers.

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Chem 162-2011 exam I + solutions - CHEM 162-2011 EXAM I...

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