what is the difference between molartiy and molality?

Asked by  | 3rd Jul, 2008, 12:14: PM

Expert Answer:

Molality is the number of moles of solute dissolved in one kilogram of solvent.

Molarity is number of moles of a solute dissolved in a litre of solution.

Molality is represented by a small "m," whereas molarity is represented by an upper case "M."

In molality, the the solvent must be weighed whereas in molarity, we measure the volume of the solvent.

For eg - To make one molal aqueous solution of sodium chloride (NaCl) , measure out one kilogram of water and add one mole of the solute (i.e. 58 gms), NaCl to it. 

1 molar solution of sodium chloride is made by placing 1 mole of a solute into a 1-liter volumetric flask. Water is then added to the volumetric flask up to the one litre mark.

Molalities are more convenient than molarities in experiments that involve significant temperature changes. Because the volume of a solution increases when its temperature increases, heating makes the solutions molarity go down- but the molality, which is based on masses rather than volumes, remains unchanged.

 

Answered by  | 3rd Jul, 2008, 05:50: PM

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