Question

What is the theoretical boiling point of 1 M NaCol at sea level (normal atmospheric pressure)? What is the freezing point of the same solution? Show calculations.

Answer 1

We know that ΔT _b = iK_bx m
Where

ΔT _b= elevation in boiling point

        = boiling point of solution - boiling point of pure solvent

        = T_b -100 ^oC

K_b = elevation in boiling point constant of water = 0.512 ^oC/m

i = vant Hoff's factor = 2 ( NaCl --> Na⁺ + Cl^-)

m = molality of the solution = 1 m

Plug the values we get T_b -100 ^oC = 2x0.512 ^oC/m x 1 m

                                                  = 1.024

                                            T_b = 1.024 +100 = 100.024 ^oC

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We know that ΔT _f = iK_f x m
Where

ΔT _f = depression in freezing point

        = freezing point of pure solvent – freezing point of solution

        = 0.0 - T_f

        = - T_f

K _f = depression in freezing constant = 1.86 ^oC/m

i= vanthoff’s factor = 2

m = molality of the solution = 1 m

Plug the values we get - T_f = 2x1.86^oC/m x 1m

                                       = 3.72 ^oC

                                     T_f = -3.72 ^oC