At a given temperature, pure A has a vaporpressure of 140 torr and pure B has a vaporpressure of 60 torr. At this temperature, wemake a solution in which the mole fractionsof A and B are equal.We then allow thissolution to reach equilibrium with its vapor.Which of the following statements about thecomposition of the VAPOR is TRUE? 4. The mole fraction of A in the vapor is 70 torr. 5. This question cannot be answered with- out knowing the molecular weights of A and B. 6. Equal numbers of moles of A and B are present in the vapor. 7. All of the other answers are true. 8. The mole fraction of A in the vapor is less than 0.5. 9. All of the other answers are false. Explanation: For CH 2 Cl 2 , P 0 = 133 torr MW = 85 g / mol For CH 2 Br 2 , P 0 = 11 torr MW = 174 g / mol This is a combination of Raoult’s Law and Dalton’s Law of Partial Pressures. The an- swer does not depend on what the masses are, as long as they are equal. You can choose any mass you like, but to speed up calculations, it is convenient to choose the mass the same as one of the molecular weights given, so that the number of moles for one of the components is exactly ONE. So, for argument’s sake, choose 85 g to be the mass of each of the components. That way you have: (85 g CH 2 Cl 2 ) parenleftbigg 1 mol CH 2 Cl 2 85 g CH 2 Cl 2 parenrightbigg
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- Fall '07
- Mole, Vapor pressure, Molecule, mol, Amount of substance