At 120 ° C, the vapor pressure of pure chlorobenzene (C₆H₅Cl) is 559 torr. What is the vapor pressure of a solution of 5.00 g of naphthalene (C₁₀H₈) in 50.0 g of chlorobenzene?
(Assume that naphthalene is not volatile.)

Question

Quizplus · Accepted Answer

Using Raoult's law:
P(solution) = X(chlorobenzene) * P(chlorobenzene)
where X(chlorobenzene) = moles of chlorobenzene / total moles
Total moles = moles of chlorobenzene + moles of naphthalene
Moles of chlorobenzene = (mass of chlorobenzene / molar mass of chlorobenzene) = (50 g / 112.56 g/mol) = 0.444 mol
Moles of naphthalene = (mass of naphthalene / molar mass of naphthalene) = (5.00 g / 128.17 g/mol) = 0.039 mol
Total moles = 0.444 mol + 0.039 mol = 0.483 mol
X(chlorobenzene) = 0.444 mol / 0.483 mol = 0.919
P(solution) = 0.919 * 559 torr = 514 torr

At 120 ° C, the Vapor Pressure of Pure Chlorobenzene