Determination of molecular mass of non-volatile solute from osmotic pressure
The osmotic pressure is a colligative property.For a given solvent the osmotic pressure depends only upon the molar concentration of solute but does not depend upon its nature. The following relation relates osmotic pressure to the number of moles of the solute,
According to Boyle Van't Hoff law (at conc. temp.)
According to Gaylussac Van't Hoff law (at conc. temp.)
From equation (i) and (ii)
, (Van't Hoff equation)
Here, C = concentration of solution in moles per litre
R = gas constant; T = temperature
n = number of moles of solute; V = volume of solution
m = molecular weight of solute; w = weight of solute
(iii) Conditions for getting accurate value of molecular mass are,
(a) The solute must be non-volatile.
(b) The solution must be dilute.
(c) The solute should not undergo dissociation or association in the solution.