# How would you determine molar mass from relative lowering of vapour pressure.

2.2k views

closed

How would you determine molar mass from relative lowering of vapour pressure.

+1 vote
by (46.8k points)
selected by

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

Then,  $x_B=\frac{n_B}{n_A}$

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.