Question

1. What is Osmotic pressure and how is it related to the molecular mass of a non volatile substances? 

2. What advantage the osmotic pressure method has over the elevation of boiling point method for determining the molecular mass?

Answer 1

1. Osmotic pressure: It is the pressure of the solution column that can prevent the entry of solvent molecules through a semi-permeable membrane, when the solution and the solvent are separated by the same. It is denoted by π. Its unit is mm 11g or atmosphere.

We know that, π = CRT 

where π is the osmotic pressure and R is the gas constant

π = \(\frac{n_2}{V}\) RT

where V is volume of solution per litre containing n₂ moles of solute.

By the above relation molar mass of solute can be calculated.

2. The osmotic pressure method has the advantage over other methods as pressure measurement is around the room temperature and molarity of the solution is used instead of molality The technique of osmotic pressure for determination of molar mass of solutes is particularly useful for biomolecules as they are generally not stable at higher temperature and polymers have poor solubility.