The use of molality and concentration scales

The quantity of a solid or a gas in solution is very often expressed as a molality rather than as a mole fraction. For example, most of the data on the free energy of formation of substances in aqueous solution refer to a reference state of unit molality. This is a matter of convenience, and the molality has less theoretic significance than the mole fraction. 

The molalUy of a solute is the amount of it (mols, i.e. gram formula weights ) per kg of solvent. Consider a solution containing 

Occasionally a volume concentration scale is used. If V is the volume of the above solution in m, then the concentration (mol m ) of the particular solute is defined by+ 

Let p be the density of the solution (kg m ). Then the volume V weighs pF kg and therefore 

It follows from (9 13) that the concentration of a given solution does not have the same value at two different temperatures on t What was previously called the molarity was the volume concentration expressed as mols per litre. 

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