If you've ever made ice cream with an old-fashioned churn, you've seen freezing point depression -- denoted Tf -- in action. Freezing point depression is the addition of a solute to lower the freezing point of a solution. In the example of the ice cream churn, salt is added to crushed ice which brings the solution below zero degrees Celcius and allows the sweet cream to freeze. To calculate the freezing point depression constant, or Kf, you'll need the equation: delta Tf = Kfcm where cm is the molal concentration of the solution.
Write down what you know. Since Kf is a constant, or a number that is always the same, it is often provided in a chart or table in chemistry books. If you don't see a Kf value provided in the problem, flip to the back of the book and look for a Kf table in the appendices. You may not need to calculate a value for yourself.
Rewrite the equation. If Kf isn't supplied, you need to solve for it with a revised freezing point depression equation. Divide the freezing point depression by the molal concentration so you have: Kf = delta Tf / cm.
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Insert the values for delta Tf and cm. For instance, if you have a solution with a molality of 0.455 which freezes at 3.17 degrees Celsius, then Kf would equal 3.17 divided by 0.455 or 6.96 degrees Celsius.
TL;DR (Too Long; Didn't Read)
Chemistry problems often require more than one equation. For instance, you may need to calculate molality before you can put it into the freezing point depression equation. Molality is equal to amount of the solute in moles over the mass of the solvent in kilograms.