Question

BE
11. The osmotic pressure of a dilute aqueous solution of a compound X containg 0.12 g per litre is twice the osmotic
pressure of a dilute aqueous solution of another compound Y containing 0.18g per litre. What is the ratio of the molecular
mass of X to Y if both remain in molecular form in solution?​

Answer 1

Given:

Mass of X in 1 L solution, w1 = 0.12 g

Mass of Y in 1 L solution, w2 = 0.18 g

Osmotic pressure of solution of X = 2 × Osmotic pressure of solution of Y

⇒ π1 = 2 × π2

To Find:

Ratio of molecular mass of X to Y.

Calculation:

- We can calculate molar mass from osmotic pressure by the formula:

M.wt = (w × R × T) / (π × V)

⇒ M1 / M2 = (w1/ π1) / (w2/ π2)      {∵ R,T and V are same}

⇒ M1 / M2 = (0.12/2 × π2) / 0.18/π2)

⇒ M1 / M2 = 1/3

- So the ratio of the molecular  mass of X to Y is 1 : 3.