Answer
See explanation below.
Work Step by Step
dT = kf x m x i
dT is Tn - Ts
Tn is normal freezing point of solvent
Ts is freezing point of solution
kf is a constant for the solvent
m is molality = moles solute / kg solvent
i = van't hoff factor = number of ions the solute dissociates into. if it doesn't dissociate, i = 1. examples. sugar, i = 1.. NaCl, i = 2
for urea
Tn water = 0C
kf water = 1.86 C / m... m is molality of solute
m urea = 0.25 molal
i = 1 for urea.. it doesn't dissociate
Ts = 0 C - 1.86 C/m x 0.25m x 1 = -0.53 C
