Answer to Question #139661 in General Chemistry for Harvey Jed A. Iman

Question

Calculate the freezing point depression of the given electrolytic solution:

5.7 g CaCl2 (calcium chloride) in 18.6 kg H2O (water)

Answer 1

 DT_f = iK_f m

                    DT_f = Depression in freezing point

                   K_f = Freezing Point Depression Constant

                m = molality

               i = van’t Hoff factor, for CaCl₂ i = 3

Moles of CaCl₂ = 5.7 g/110.98 g mol⁻¹ = 0.0513 mol

Mass of water in kg = 18.6 kg

Hence molality of CaCl₂ = 0.0513 mol/18.6 kg = 2.758×10^-3 mol kg^-1

 K_f for water is =1.86 K kg mol^–1

 DT_f = 3×1.86 K kg mol^–1×2.758×10^-3 mol kg^-1

        = 0.01539 K

Depression in freezing point 0.01539 K