Question

Which of the following will have the lowest freezing point and why?
a. Water with .5 moles LiNO3
b. Water with 1.0 mole Na3P

Answer 1

ΔT_f = (k_f x n x 1000)/W ……………………………(1)

Where ΔT_f = Depression in freezing point

Kf = molal freezing constant

N = no. of moles of the solute

W = mass of the solvent (water)

Putting values in eq (1)

1. Water with .5 moles LiNO3

ΔT_f = (k_f x 0.5 x 1000)/W = 500 k_f/W………………..(2)

        b. Water with 1.0 mole Na3P

ΔT_f = (k_f x 1.0 x 1000)/W = 1000 k_f/W………………..(3)

The solution Water with 1.0 mole Na3P will have the lowest freezing point as depression in the freezing point (ΔT) is directly propotional to the no. of moles of the solutes so, as no. of moles are more , more is the value of ΔT. Water freezing point is 273K hence eq (3) has the lowest freezing point.