Question

Calculate the solidification point of a 30% solution of CH3OH (MW = 32 G / MOL) in water (MW = 18 G / MOL)
KC of water = 1.86 ºC / m

Answer 1

solidification point = freezing point

Apply Colligative properties

This is a typical example of colligative properties.

Recall that a solute ( non volatile ) can make a depression/increase in the freezing/boiling point via:

dTf = -Kf*molality * i

where:

Kf = freezing point constant for the SOLVENT; Kb = boiling point constant for the SOLVENT;

molality = moles of SOLUTE / kg of SOLVENT

i = vant hoff coefficient, typically the total ion/molecular concentration.

At the end:

Tf mix = Tf solvent - dTf

Tf mix = Tf solvent -Kf*molality * i

since it is molecular, i = 1,

molality = mol of solute / kg of solvent

assume a basis of 100 g of mix

30 g --> methanol

70 g --> water

mol of methanol = mass/MW = 30/32 = 0.9375

kg of solvent (/ater) = 70*10^-3 kg

molality = mol of methanol / kg of solvent = 0.9375/( 70*10^-3 )= 13.3928

Tf mix = Tf solvent -Kf*molality * i

Tf mix = 0-1.86*13.3928* 1

Tf mix = -24.91°C