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Look up the freezing point of the liquid (solvent) for which you are calculating the new freezing point. You can find the freezing point of any chemical on the material safety data sheet that accompanies it. For example, water has a freezing point of 0 degrees Celsius.
Calculate the molal concentration of the solution that will be created after you add your dissolved substance (solute) to the solvent. For instance, consider a solution created by dissolving 0.5 moles of salt into 1 liter (L) of water. One liter of water has a mass of 1 kilogram (kg), so:
Molality = moles of solute / mass of solvent = 0.5 / 1 = 0.5 m
You can obtain the moles of your solute by dividing the number of grams dissolved by its molecular mass (see Resources).
Look up the freezing point depression constant (K) for the solvent you are using. A freezing point depression constant is an experimentally determined number that indicates the degree to which a change in a liquid's solute concentration affects its freezing point. Water has a freezing point depression constant of 1.86.
Plug your values into the following equation to calculate the new freezing point of your solution:
Freezing point = old freezing point - K x molality
Our water example would look like this:
Freezing point = 0 - 1.86 x 0.5 = -0.93 degrees Celsius