Question

If 20 g of a solute was dissolved in 500 mL of water and osmotic pressure of the solution was found to be 600 mm of Hg at ${15}^{o}C$ , then molecular weight of the solute is

• A. 1000
• B. 1200
• C. 1400
• D. 1800

Answer 1

Answer: (B)

Key Idea: Osmotic pressure is colligative property. Use the formula $\pi V=nRT$ to solve problem. $\pi V=nRT$ or $\pi =\frac{n}{V}RT=\frac{mRT}{MV}$ where $\pi$ = osmotic presure = 600 mm of Hg $=600/760atm$ m = mass of solute = 20 g M = molecular mass of solute R = 0.0821 L atm $mo{l}^{-1}{K}^{-1}$ $T={15}^{o}C=15+273=288K$ $V=500mL=0.5L$ $\frac{600}{760}=\frac{20\times 0.0821\times 288}{M\times 0.5}$ or $M=1200$ $\therefore$ Molecular mass of solute is 1200.