Question

27.Calculate the change in freezing point when 116.8 grams of barium chloride are added to 250 grams of water. Kf = 1.86°C/m; Kb = 0.512°C/m
A. -4.17°C
B. -12.5°C
C. -8.35°C
D. 0°C

Answer 1

Freezing point depression, ΔT, is calculated using the formula:

ΔT= - Kf * molal

Where molal is the molality of the solute (moles solute/kg solvent)

In this case the solute is Barium Chloride while the solvent is Water. The formula for molality is:

molal = (mass of solute / molar mass solute) / (mass of solvent)

molal = (116.8 g / 208.23 g/mol) / (0.25 kg Water) = 

molal= 2.24 mol/kg = 2.24 m

Substituting to the 1st equation:

ΔT= - 1.86 °C/m (2.24 m)

ΔT= -4.17 °C

So the answer is A.