Answer to Question #320465 in General Chemistry for Nja

Question

A solution of propylene glycol (antifreeze) in the car's radiator has a freezing point of -20°C. What is the molality of the solution?

Answer 1

ΔT_f = K_f * m

K_f - molal freezing point depression constant or cryoscopic constant

m - molality of the solution

ΔT_f = T_f⁰ - T_f^s

T_f⁰  represents the freezing point of the pure solvent

T_f^s  represents the freezing point of the solution

ΔT = T(H₂O) - T(C₃H₈O₂)

T(C₃H₈O₂) = -20°C = 253.15K

ΔT = 273K - 253.15K=19.85K

K(H₂O) = 1.86 K*kg/m

m = ΔT / K(H₂O) = 19.85 K/ 1.86 K*kg/m = 10.67 kg/m