JEE MAIN - Chemistry (2024 - 8th April Morning Shift - No. 26)
A solution containing $$10 \mathrm{~g}$$ of an electrolyte $$\mathrm{AB}_2$$ in $$100 \mathrm{~g}$$ of water boils at $$100.52^{\circ} \mathrm{C}$$. The degree of ionization of the electrolyte $$(\alpha)$$ is _________ $$\times 10^{-1}$$. (nearest integer)
[Given : Molar mass of $$\mathrm{AB}_2=200 \mathrm{~g} \mathrm{~mol}^{-1}, \mathrm{~K}_{\mathrm{b}}$$ (molal boiling point elevation const. of water) $$=0.52 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$$, boiling point of water $$=100^{\circ} \mathrm{C} ; \mathrm{AB}_2$$ ionises as $$\mathrm{AB}_2 \rightarrow \mathrm{A}^{2+}+2 \mathrm{~B}^{-}]$$
Explanation
To find the degree of ionization $$(\alpha)$$, we need to use the boiling point elevation formula and the van't Hoff factor. The formulas we will use are:
$$\Delta T_b = i \cdot K_b \cdot m$$
where:
$$\Delta T_b$$ is the boiling point elevation,
$$i$$ is the van't Hoff factor,
$$K_b$$ is the molal boiling point elevation constant, and
$$m$$ is the molality of the solution.
Given data:
- $$\Delta T_b = 100.52^{\circ} \mathrm{C} - 100^{\circ} \mathrm{C} = 0.52^{\circ} \mathrm{C}$$
- Mass of $$\mathrm{AB}_2$$ = 10 g
- Mass of water = 100 g = 0.1 kg
- Molar mass of $$\mathrm{AB}_2$$ = 200 g/mol
- $$K_b$$ = 0.52 K kg mol-1
First, we calculate the molality (m):
$$m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}}$$
$$= \frac{\frac{10}{200}}{0.1} \:\mathrm{mol\ kg}^{-1}$$
$$= 0.5 \:\mathrm{mol\ kg}^{-1}$$
Next, using the formula for boiling point elevation:
$$0.52 = i \cdot 0.52 \cdot 0.5$$
$$i = \frac{0.52}{0.52 \cdot 0.5}$$
$$i = \frac{1}{0.5}$$
$$i = 2$$
Now, the van't Hoff factor $i$ is related to the degree of ionization $$(\alpha)$$. For the electrolyte $$\mathrm{AB}_2$$, which ionizes as $$\mathrm{AB}_2 \rightarrow \mathrm{A}^{2+}+2\mathrm{B}^{-}$$, the van't Hoff factor $i$ can be expressed as:
$$i = 1 + (n-1)\alpha$$
where
- $$n$$ is the number of ions produced (which is 3 for $$\mathrm{AB}_2 \rightarrow \mathrm{A}^{2+}+2\mathrm{B}^{-}$$)
- $$\alpha$$ is the degree of ionization
Substituting $i = 2$ into the equation:
$$2 = 1 + 2 \alpha$$
$$2 - 1 = 2 \alpha$$
$$\alpha = \frac{1}{2}$$
$$\alpha = 0.5$$
The degree of ionization $$(\alpha)$$ is therefore
$$0.5 \times 10^{-1} = 5 \times 10^{-2}$$.
Hence, the degree of ionization of the electrolyte $$(\alpha)$$ is approximately 5 $$\times 10^{-1}$$ (the nearest integer is 5).
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