In: Chemistry
At –12.5 °C (a common temperature for household freezers), what is the maximum mass of saccharin (C7H5NO3S) you can add to 1.00 kg of pure water and still have the solution freeze? Assume that saccharin is a molecular solid and does not ionize when it dissolves in water. Kf values are given here.
Solvent | Formula | Kf
value*
(°C/m) |
Normal freezing point (°C) |
Kb
value
(°C/m) |
Normal boiling point (°C) |
water | H2O | 1.86 | 0.00 | 0.512 | 100.00 |
benzene | C6H6 | 5.12 | 5.49 | 2.53 | 80.1 |
cyclohexane | C6H12 | 20.8 | 6.59 | 2.92 | 80.7 |
ethanol | C2H6O | 1.99 | –117.3 | 1.22 | 78.4 |
carbon tetrachloride |
CCl4 | 29.8 | –22.9 | 5.03 | 76.8 |
camphor | C10H16O | 37.8 | 176 |
We know the equation for the freezing point depression is:
T = kf*m
where,
T is the depression in the freezing point
kf is the molal freezing point depression constant
m is the molality of the solute i.e. moles per kg of a solute
Given T = 12.5oC, kf = 1.86
Therefore molality m = T/kf = 12.5/1.86 = 6.72 moles saccharin per kg of water
This also is equal to 6.72 * MMsaccharin = 6.72*183.18 = 1231g Saccharin per kg water.
Below this amount the water will still freeze at the given temperature.