Question

A solution of 0.90 g of an unknown nonelectrolyte in 100 ml of water at 27◦c has an osmotic pressure of 0.55 atm. what is the molecular weight of this compound? (assume

Answer 1

Osmotic pressure is mathematically expressed as
π = CRT
where,  C = concentration of solution
R = gas constant = 0.082 atm mol-1 K-1
T = temperature = 300 K
π = osmotic pressure = 0.55 atm

∴ 0.55 = C X 8.314 X 300
∴C = 0.022 M

Now, conc of solution (in terms of molarity) = $\frac{\text{weight of solute}}{\text{molecular weight X volume of solution(l)}}$

Given: weight of solute = 0.9 g
volume of solution = 100 ml = 0.1 l

∴ 0.022 = $\frac{0.9}{\text{molecular weight X 0.1}}$
∴ Molecular weight = 4.09 g/mol