Question

Ethylene glycol (62.1 g/mol), a nonvolatile organic compound, is commonly used in antifreeze. If you buy pure ethylene glycol and mix your own radiator fluid by pouring 2.5L of the ethylene glycol in a bucket with 2.5L of water, what will be the freezing point (°C)?

van ’t Hoff factor of ethylene glycol = 1
density of ethylene glycol = 1.11 g/mL
density of water = 1.00 g/mL
Kf for water = 1.86°C kg/mol

Answer 1

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

dTb = Kb*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

Tb mix = Tb solvent - dTb

a)

mass of EG = D*V = 1.11*2.5*10^3 = 2775 g

mass of water = 2.5 L = 2.5 kg

b)

mol of EG = mass/MW = 2775/62.07 = 44.707 mol of EG

c)

molality = mol of EG / kg water = 44.707/2.5 = 17.8828 molal

d) Freezing point

Tf = 0 - Kf*molal = 0 - 1.86*17.8828

Tf = -33.26 °C