Boiling Point Elevation of Solutions

grams of solute dissolved, s

20.

grams of solvent, m

120.

time, t

10.

solutes

sodium chloride

boiling point of

sodium chloride

solution =

276.138

K

change in boiling point =

2.63827

K

vapor pressure =

4.35533

mmHg

When a solute is added to a solvent, the boiling point of the resulting solution increases, according to the equation

T=Kmi

, where

K

is the ebullioscopic constant (specific to each solvent),

m

is the molality (moles solute over kilograms solvent), and

i

is the van't Hoff factor, which depends on the extent to which the solute ionizes in solution. A solution boils when the vapor pressure is equal to the atmospheric pressure. When adding a solute to a solvent, the vapor pressure of the resulting solution is lower than the vapor pressure of the original solvent, so it takes a higher temperature for the vapor pressure to equal atmospheric pressure, ultimately resulting in an elevated boiling point. This Demonstration shows the boiling point elevation from solute and solvent data. A graphic approximating the progress of the dissolution process with time is also shown.