Consider a solution formed by dissolving 40.84 g of an unknown, nonvolatile, mol

Question

Consider a solution formed by dissolving 40.84 g of an unknown, nonvolatile, molecular solute in enough cyclohexane (a nonpolar, organic solvent) to produce a final volume of 250.0 mL. If the final solution mass is 219.27 g and its freezing point for the solution is measured to be -23.15 degree C; based on the freezing point depression relative to pure cyclohexane, calculate the molar mass of the solute, given that the freezing point-depression constant for cyclohexane is 20.2 degree C kg/mol and its freezing point is 6.55 degree C Calculate the vapor pressure produced by this solution at 25.0 degree C, given that the vapor pressure of cyclohexane is 98.7 torr at the same temperature.

Explanation / Answer

m = 4.84 g of unkown

Vfinal = 250 mL

Q1.

m = 219.27 g

mass of solvent = 219.27-40.84 = 178.43 g = 0.17843 kg

Tf = -23.15°C

MW of solute if

Kf = 20.2 and Tf = 6.55

so

dTf = 6.55-23.15 = - 16.6°C

dTf = -Kf*m

-16.6 = -20.2*mol/0.17843

mol = mass/MW

-16.6 = -20.2*mass/MW/0.17843

-16.6 = -20.2*40.84/MW/0.17843

Mw = (-20.2*40.84)/(0.17843)/(-16.6 ) = 278.5230 g/mol

Q2

P° P = 98.7

mol = mass/MW = 40.84/278.5230 = 0.1466

mol of cyclohexane = 178.43 /84.16 = 2.120

x solute = 0.1466 / (0.1466 + 2.120)= 0.0646

so

dP = xsolute¨*Psolvnet

dP = 0.0646*98.7 = 6.37602 torr

Pmix = 98.7-6.37602

Pmix = 92.32398 torr

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