Question

Addition of non-volatile solute to a solvent always increases the colligative properties such as osmotic pressure, $\Delta P,\Delta T_b$ and $\Delta T_f$. All these colligative properties are directly proportional to molality if solutions are dilute. The decrease in colligative properties on addition of non-volatile solute is due to increase in number of particles.
For different aqueous solutions of $0.1N$ urea, $0.1NNaCl,0.1NN{a}_{2}S{O}_4$ and $0.1NN{a}_{3}P{O}_4$ solution at $27^{\circ }C$ the correct statements are :
1. The order of osmotic pressure is $NaCl>N{a}_{2}S{O}_4>N{a}_{3}P{O}_4>$ urea
2. $\pi =\frac{\Delta T_b}{K_b}\times ST$ for urea solution
3. Addition of salt on ice increases its melting point
4. Addition of salt on ice brings in melting of ice earlier

• A. 2, 3, 4
• B. 1, 2, 4
• C. 1, 2, 3
• D. 3, 4

Answer 1

Answer: (B)

1. Molar concentrations are $0.1MNaCl$, $\frac{0.1}{2}MN{a}_{2}S{O}_4$ and $\frac{0.1}{3}MN{a}_{3}P{O}_4$
$\therefore \pi \propto CX(1-\alpha +X\alpha +Y\alpha )\propto C(X+Y)$ if $\alpha =1$
$\therefore {\pi }_{\text{urea }}\propto 0.1\times 1,{\pi }_{NaCl}\propto 0.1\times 2;{\pi }_{N{a}_{2}S{O}_4}$ $\propto \frac{0.1}{2}\times 3;{\pi }_{N{a}_{3}P{O}_4}\propto \frac{0.1}{3}\times 4$
2. Also $\pi =CM\times S\times T$ and $\Delta T_b=$ Molality $\times K_b$ If Molarity $=$ Molality (for dilute solution) $\pi =\frac{\Delta T_b}{K_b}\times S\times T$
3. Addition of salt on ice lowers the freezing point.
4. Also addition of salt on ice lowers its melting point and thus, ice melts earlier