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 =
