Question

How will you determine the molecular mass of a substance by osmotic pressure method?

Answer 1

For n moles of the solute dissolved in V litres of the solution, van’t Hoff equation can be written as

πV = nRT    ...(i)

If w grams of a solute are dissolved in V litres of a solution and M' is the molecular mass of the solute, we have

`n = w/(M')`

Putting the value of n in the equation (i), we have

`pi V = w/(M') RT`

or `M' = (w R T)/(pi V)`    ...(ii)

Thus, the molecular mass of the solute can be calculated with the help of Eq. (ii) provided the osmotic pressure π of the solution, mass w of solute dissolved in V litres of the solution and temperature T are known. The value of R is taken to be equal to 0.0821 L atm K⁻¹ mol⁻¹ when π is expressed in atmospheres, volume in litres and T in Kelvin.