Question

7. What is the mass percent of ethylene glycol (HOCH2CH2OH) in a solution of ethylene glycol in water that has a freezing point of –19.2°C? (Kf for water is 1.858°C/m.) A) 99.8% B) 39.1% C) 64.1% D) 60.1% E) 10.3%

Answer 1

freezing point depression formula

Delta T_f = (T_f0 - T_f) = i K_f m

where:

T_f = Freezing Point of the solvent in the solution

T_f0 = Freezing Point of the pure solvent

i = i factor which will be 1 if the solute is a non-electrolyte

K_f = Freezing Point Depression Constant

m = concentration in moles of solute per kilogram of solvent ( molality)

so 19.2= 1 x 1.858 x m

m= 19.2 / 1.858 = 10.33 moles / kg of solvent

1.033 moles of glycol / 100 g of solvent

Molar mass of ethylene glycol = 62.068 g/mol

mass of ethylene glycol / 100 g of solution = 1.033 * 62.068

Mass percent = 64.1g %w/w

Answer is C