• Molarity (M) is defined as number of moles of solute dissolved in one litre (or one cubic decimetre) of solutio


  • where WB is the mass of solute and MB is the molar mass of the solute.
  • For dilution; where M1 is initial molarity, V1 is initial volume, M2 is final molarity and M2 is final volume.
  • M1V1 = M2V2
  •  are called molar, decamolar, decimolar, pentimolar, semimolar and centimolar solution respectively.


  • Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent and is expressed as:

  • Molality (m) = (Given mass / molecular mass) * ( 1/mass of solvent in Kg )
  • Its SI unit is moles / Kg
  • It is represented as m or molal solution.

Difference between Molarity and Molality



The molarity of a given solution is defined as the total number of moles of solute per litre of solution.

Molality is defined as the total moles of a solute contained in a kilogram of a solvent.

Mathematical expression is

M = number of moles of the solute / Volume of solution given in terms of litres.

The mathematical expression

m = Number of moles of solute / Mass of solvent in Kg


Depends on the volume of the whole solution.

Depends on the mass of the solvent.

Unit sign expressed as (M).

Unit sign expressed as (  ).

Molarity has a unit of moles/litre.

Molality has a unit of moles / kg

 Vapour density

  • Vapor density is the ratio of the mass of a volume of a gas, to the mass of an equal volume of hydrogen, measured under the same conditions of temperature and pressure.
  • It is a unitless quantity.

Relation between Vapour density and Molecular weight

  • Molecular weight = 2 X Vapour density


  • Normality (N) It is defined as the number of g-equivalents of solute per litre of solution or as the number of mg-equivalents of a substance per millilitre of solution.


  • If specific gravity is known, normality is calculated as