Define the following terms: (i) Mole fraction (ii) Ideal solution. 15.0 g of an unknown molecular material was dissolved in 450 g of water. The resulting solution was found to freeze at −0.34 °C. What is the molar mass of this material?

(a) (i) The ratio of the number of moles of a solute (components of a mixture) to the total number of moles in the mixture is called mole fraction.
xA = nA/(nA + nB) or xB = nB/(nA + nB)
(ii) A solution that obeys Raoult's law at all temperatures and concentrations is called an ideal solution.

Given : W2 = 15.0 g, ΔTf = 0.34 °C
W1 = 450 g, Kf = 1.86 K kg mol−1

(b) From the formula,
ΔTf = 1000 × Kf × W2 / (W1 × M2)
M2 = 1000 × Kf × W2 / (ΔTf × W1)
= 1000 × 1.86 × 15 / (0.34 × 450)
M2 = 182.35 g/mol