Liquid Solution

Step 1: Find the decrease in freezing point
The difference between the normal freezing point of water and the solution's freezing point is called the depression in freezing point. The calculation is:
‌∆Tf=Tf0−Tf=273.16−272.814
‌∆Tf=0.346K
Step 2: Use the freezing point depression formula
The formula is: ∆Tf=Kf⋅m where m is molality.
m=‌

∆Tf

Kf

=‌

0.346

1.86

=0.186mol∕kg
Step 3: Calculate the moles of solute
Molality tells us moles of solute in 1 kg of solvent (water). Here, amount of water is 21.5g=0.0215kg.
Moles of solute =0.186×0.0215=0.00400mol
Step 4: Calculate the molar mass
Molar mass =‌

‌ mass of solute ‌

‌ moles of solute ‌

Molar mass =‌

0.2

0.00400

=50g∕mol