Question

A. Explain why a 1.0m solution of NaCl in water freezes at a lower temperature than pure water.

   B. Explain why a 1.0m CaCl₂ solution in water freezes at a lower temperature than a 1.0m solution of NaCl in water.

Answer 1

A)
Adding any solute to the solvent will dilute the solvent molecules, and according to Raoult's law, this leads to a decrease in vapor pressure. The lowering of the vapor pressure leads to the lowering of the temperature at which the vapor pressures of the liquid and frozen forms of the solution will be equal. Thus the freezing point is lowered

the decrease in freezing point is written as,
ΔTf = i*Kf*m
i is number of particle that is formed from solute

B)
i of CaCl2 formed = 3
CaCl2 ---> Ca2+   + 2Cl-

i of NaCl formed = 2
NaCl ---> Na+ + Cl-

if there is larger value for i, depression in freezing point will be large
this is why a 1.0m CaCl2 solution in water freezes at a lower temperature than a 1.0m solution of NaCl in water