The freezing point of pure benzene was measured as 5.49°C. With the same benzene and the same equipment, the freezing point of a solution containing 6.60 g of an unknown molecular compound in 140 g of benzene was found to be 2.35°C. The cryoscopic constant, Kf, for benzene is 5.12 deg-kg/mol. Calculate the mole weight of the unknown.

Question

Quizplus · Accepted Answer

Firstly, we need to calculate the change in freezing point for the solution:
ΔTf = Kf·m
where ΔTf = Tfpure - Tfsolution, Kf = cryoscopic constant, and m = molality of the solution
ΔTf = 5.49 - 2.35 = 3.14°C
m = (moles of solute) / (mass of solvent in kg)
We are given the mass of solvent to be 140 g = 0.14 kg, so we need to find the moles of solute:
moles of solute = 6.60 g / (molecular weight of solute)
Now we can solve for the molecular weight of the solute:
3.14 = 5.12 · [(6.60 / (molecular weight of solute)) / 0.14]
0.14 · 3.14 · (molecular weight of solute) / 6.60 = 5.12
molecular weight of solute = 76.9 g/mol
Therefore, the answer is choice B: 76.9 g/mol.

The Freezing Point of Pure Benzene Was Measured as 5