What is the molar mass of 35 g of an unknown substance that depresses the freezing point of .350 kg of water .

Answer 1

Look up the colligative properties of water. Look up water's freezing point depression/molal. You know the mass of solute, you know the observed fp depression, you therefore know the molality of solution. If you know the molality and the volume of solvent you then know the solute's molar mass... **IF** it is molecular (e.g, sugar) and not dissociative ionic (e.g., table salt). Horribly phrased question.

Answer 2

You cannot answer this question without knowing how many degrees the freezing point was depressed.

Once you know that, use the formula

delta T = Kf m (m = molals)

delta T = the freezing point depression

Kf for water = -1.86 0C / Kg molals

molal = moles of solute / kg solvent

kg solvent = 0.350 kg.

mass = 35 grams

molar mass = unknown.

Once you have the delta T, put everything in and solve for molar mass