What is the freezing-point depression of a solution that contains 0.705 mol of a nonelectrolyte solute in 5.02 kg of water? (Kf = 1.86 °C/m)

Question

Although I understand Chemistry and have been doing well in the class, I just do not understand this problem. Thank you for any help!

Answer 1

Freezing point depression is equal to the molal freezing point depression constant (Kf) times the molality of the solution. 

 

deltaTf = Kf x m

 

Since Kf was given, we just need to find the molality of the solution and then substitute everything into the equation. Molality is equal to moles of solute divided by kilograms of solvent. 

 

m = moles solute/kg solvent

    = 0.705 moles solute/5.02 kg solvent

    = 0.140 m 

deltaTf = Kf x m

            = (1.86°C/m)(0.140m) 

            =  0.261°C

Therefore the freezing point depressed by 0.261°C.