Question

a solution is prepared by mixing 2.17g of an unknown non-electrolyte with 225.0g of chloroform. The freezing point of the resulting solution is -64.2 C . The freezing point of pure chloroform is -63.5 C and its kf= 4.68C m^-1 . What is the molecular mass of the unknown?
i know we must use molality and freezing point depression formula, but i do not know in what order.

Answer 1

When I come up with that dilemma I put doen the formula I need, plug in the values, see which unknown I don't have and work it from equation to equation. Here is the order you need.

delta T = Kf*m.
You have delta T (or can calculate it) and Kf so solve for m.

molality = mols/ kg solvent.
You have m and kg solvent, solve for mols.

Then mols = grams/molar mass. You have mols and grams, solve for molar mass.