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      Freezing Point Depression

DTf = -kfmA

Example. What is the freezing point of ocean water containing approximately one NaCl per 100 water molecules, or one mole of NaCl per 100 moles of H2O as described previously in this chapter?


Colligative properties were the basis for one of the original proofs that salts really did dissociate, and for determining how many ions were produced. For example, one could imagine that the compound potassium ferricyanide, K3Fe(CN)6 dissociates in solution as follows:

Is this true? A freezing-point experiment can settle the matter.

Example. When 300 mg of potassium ferricyanide are dissolved in 10 ml of water, the freezing point falls to -0.68° C. How many ions are produced by one K3Fe(CN)6 formula unit?


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