Boiling points.... finding mass...?

Question

1.78 grams of an unknown solute is dissolved in 22.43 grams of a solvent having a K(sub f) of 7.11 C/m. The normal freezing point of the solvent is 48.91 C. The solution freezes at 46.38 C. Find the molecular mass of the unknown.

All I need is an equation to solve for this problem, thanks!

Answer 1

Freezing point depression is based on molality, e.g. moles of solute / kg. solvent. In your problem, the freezing point is depressed 2.53 C. Based on the factor of 7.11 C/m, you have 2.53 / 7.11 = 0.356 molals.

So 1.78 g / MW of the unknown = moles of unknown.

moles of unknown / .02243 kg. = 0.356 molals.

You can easily solve for the MW of the unknown.