Question

What is the freezing point of an aqueous solution that boils at 107.0 {\rm ^\circ {\rm C}} ?

Express your answer using two significant figures.

Answer 1

we know that

elevation in boiling point is given by

dTb = Kb x m

also

dTb = boiling point of solution - boiling point of pure water

in this case

dTb = 107 - 100

dTb = 7

also

depression in freezing point is given by

dTf = Kf x m

so

we get

dTf / dTb = Kf / Kb

we knwo that

Kf for water = 1.86

Kb for water = 0.512

so

dTf / 7 = 1.86 / 0.512

dTf= 25.43

now

dTf = freezing point of water - freezing point of solution

so

25.43 = 0 - freezing point of solution

freezing point of solution = -25.43 C

so

freezing point of the solution is -25.43 C