Question

In: Chemistry

At –12.5 °C (a common temperature for household freezers), what is the maximum mass of saccharin...

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

Solutions

Expert Solution

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.


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