Question

A solution containing 4.50 g of a non electrolyte (i=1) dissolved in 125 g of water

freezes at -0.372ºC. Calculate the molar mass of the solute.

Step 1: First compute the molality of the freezing point equation.

Step 2. Then, from the definition of molality, compute the number of moles

solute, n(solute), in the sample.

Step 3. Solve for the molar mass.

Answer 1

$\Delta T_{f}=i*m*K_{f}$

where $\Delta T_{f}$ is depression in freezing point.

m is molality of the solution.

$K_{f}$ is molal freezing point depression constant

calculating m now,

$m = \frac{w_{solute}}{M_{solute}*w_{solvent}(in kg)}$

$\Delta T_{f} =0.372\degree C$

$\Delta K_{f}= 1.86\degree C/m$

From here we can calculate m, which turn outs to be 0.2 m.

from here using Molality formula, $M_{solute}=$ 180 g.