Molarity and molality

These two similar-sounding terms must not be confused. The molarity of a solution is the number of moles (gram molecular weights) of solute in 1 litre (1 dm3) of solution. The molal-ity is the number of moles of solute in 1 kg of solvent. Molality has the unit, mol kg1, which is an accepted SI unit. Molarity may be converted to SI units using the relationship 1 mol litre 1 = 1 mol dm 3 = 103 mol m 3. Interconversion between molarity and molal-ity requires a knowledge of the density of the solution.

Of the two units, molality is preferable for a precise expression of concentration because it does not depend on the solution temperature as does molarity; also, the molality of a component in a solution remains unaltered by the addition of a second solute, whereas the molarity of this component decreases because the total volume of solution increases following the addition of the second solute.

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