Colligative Properties and Determining Molar Mass

Because of this peculiarity, the colligative properties can be used to determine the amount of substance Wb of sample of an unknown substance B and, therefore, if the mass of the sample is known, also the molar mass Mb = m /n. Let us take a quick look at this by considering the example of lowering of freezing point. Xb ns/riA is valid at high dilution, and because of a = rriA/MA, we have Xb Wb MA/rriA- The quotient n- IrriA corresponds to the molality b- (compare Sect. 1.5). Inserting these expressions in Eq. (12.14) for the freezing-point depression results in 

There are analogous relaticMis and applications for the raising of boiling point  

The ebullioscopic constant b that corresponds to kf is positive, and because of its higher denominator (Avap A Afus A), it is (in absolute terms) smaller than kf 

The gas law also belongs to the colligative properties, but often it is not mentioned in this context. It can be used for the same purposes. The vacuum appears here in the role of the solvent, whereas the gas pressure corresponds to the osmotic pressure. 

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