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

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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.

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