Answer to Question #68973 in General Chemistry for Elizabeth

Question

Calculate the freezing point of 0.10 m NaCl. The freezing point depression constant for water is 1.86 °C kg mol-1. Hint: Don’t forget that when NaCl dissolves, it dissociates into Na+ and Cl- ions, resulting in two moles of solute per mole of NaCl dissolved.

Answer 1

ΔT = iKfm
ΔT = change in temperature, °C;
i = van 't Hoff factor;
Kf = molal freezing point depression constant or cryoscopic constant in °C kg/mol;
m = molality of the solute in mol solute/kg solvent.
m (NaCl) = moles of NaCl/kg water
ΔT = 2 x 1.86 °C kg/mol x 0.1 mol/kg = 0.372 °C
Therefore, freezing point of 0.1 m NaCl solution is -0.372 °C.