1. The measurement of relative lowering of vapour pressure can be used to determine the molar mass of a non-volatile solute.
2. A known mass of the solute is dissolved in a known quantity of solvent. The relative lowering of vapour pressure is measured experimentally.
3. According to Raoult’s law, the relative lowering of vapour pressure is
WA = weight of solvent
WB = weight of solute
MA = Molar mass of solvent
MB = molar mass of solute
where, nA = number of moles of solvent
nB = number of moles of solute.
For dilute solution, nA > > nB , nA + nB = nA
Number of moles of solvent and solute arc
From the above equation, molar mass of the solute MB can be calculated using the known values of WA , WB , MA and the measured relative lowering of vapour pressure.