Osmotic pressure measurements are less accurate and difficult to carry out. Therefore, this method is not useful for measuring molecular weights. But it is used to find out the molecular weights of polymers, e.g., plastics, proteins, and starch having many monomer units. Knowing the value of the osmotic pressure of a given solution, the molecular weight of the solute can be calculated using van't Hoff equation.

An aqueous solution of salt contains \(20 g\) of \(NaCl\) per liter. The pressure of the solution at \(35^oC\) will be

\(8.622 atm\)

Given,

Concentration, \(C = 20 g\) of \(NaCl\) per liter

or, \(C = \frac{20}{58.44}mol/L\)

Temperature, \(T = 35^oC \text{ = }35 + 273 = 308 K\)

Gas constant, \(R = 0.0821 L\text{ atm K}^{-1}\text{ mol}^{-1}\)

We know, for osmotic pressure,

\(\pi = CRT\) where, [\(\pi \) is osmotic pressure ]

or, \(\pi = \frac{20}{58.44} × 0.0821 × 308\)

or, \(\pi ≈ 8.622 atm\)