The boiling point of impure water varies. It can be determined by multiplying the molal concentration of the impurity, the molal boiling point elevation constant of water, and the Van't Hoff factor of the impurity. This value is then added to the boiling point of pure water.
The molal boiling point elevation of water is 0.52 degrees Celsius per molal, and the boiling point of pure water is 100 degrees Celsius. Sodium chloride has a Van't Hoff factor of two, so if one molal of sodium chloride is added to pure water, the boiling point of the impure water will be 101.4 degrees Celsius. This was determined by the calculation (1 x 0.52 x 2) + 100.