Question

A 4.305-g sample of a nonelectrolyte is dissolved in 105 g of water. the solution freezes at -1.23c. calculate the molar mass of the solute. kf for water = 1.86c/m.

Answer 1

The answer is 62.00 g/mol. 
Solution: 
Knowing that the freezing point of water is 0°C, temperature change Δt is 
     Δt = 0C - (-1.23°C) = 1.23°C 
Since the van 't Hoff factor i is essentially 1 for non-electrolytes dissolved in water, we calculate for the number of moles x of the compound dissolved from the equation 
     Δt = i Kf m 
     1.23°C = (1) (1.86°C kg mol-1) (x / 0.105 kg) 
     x = 0.069435 mol 
Therefore, the molar mass of the solute is  
     molar mass = 4.305g / 0.069435mol = 62.00 g/mol