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