JEE Chemistry 2025 Question with Solution

Given:

• For $\text{AX}_2$: mass of solute $= 1.24 \, \text{g}$, molar mass $= 124 \, \text{g mol}^{-1}$, mass of water $= 1 \, \text{kg}$, boiling point $= 100.105^\circ \text{C}$
• For $\text{AY}_2$: mass of solute $= 2.54 \, \text{g}$, molar mass $= 250 \, \text{g mol}^{-1}$, mass of water $= 2 \, \text{kg}$, boiling point $= 100.026^\circ \text{C}$
• $K_b = 0.52 \, \text{K kg mol}^{-1}$

Find: Which statement about ionisation is correct.

Use the boiling point elevation relation:

$$\Delta T_b = i K_b m$$

where $i$ is the van't Hoff factor and $m$ is the molality.

For $\text{AX}_2$:

$$m = \frac{1.24/124}{1} = 0.01 \, \text{mol kg}^{-1}$$ $$\Delta T_b = 100.105 - 100 = 0.105^\circ \text{C}$$

So,

$$0.105 = i \times 0.52 \times 0.01$$

The provided the solution identifies the correct option as A.

For $\text{AY}_2$:

$$m = \frac{2.54/250}{2} = 0.00508 \, \text{mol kg}^{-1}$$ $$\Delta T_b = 100.026 - 100 = 0.026^\circ \text{C}$$

So,

$$0.026 = i \times 0.52 \times 0.00508$$

the solution again marks A as the correct option.

Verification: the solution contains inconsistent arithmetic and conflicting textual conclusion, but the page explicitly states The Correct Option is A. Therefore, taking the solution, the correct option is A.

Therefore, $\text{AX}_2$ and $\text{AY}_2$ are both fully ionised according to the solution.

Given: Same data as above.

Find: Whether the source working is internally consistent.

For a solute of the form $\text{AX}_2$ or $\text{AY}_2$, complete ionisation would produce 3 ions, so the ideal van't Hoff factor for full ionisation is $i = 3$.

The source HTML shows two different numerical interpretations:

1. One place explicitly says The Correct Option is A.
2. The written conclusion in both approaches says $\text{AX}_2$ is completely unionised while $\text{AY}_2$ is fully ionised, which corresponds to D.

Also, the arithmetic shown in the source is inconsistent. For example,

$$\frac{0.105}{0.52 \times 0.01} = \frac{0.105}{0.0052}$$

which is not equal to $1$, and

$$\frac{0.026}{0.52 \times 0.00508}$$

is also computed inconsistently in the source.

Because the instructions require the solution to be the primary source, and the page explicitly labels the correct option as A, the extracted answer is A despite the mismatch with the displayed calculations.

Therefore, the extracted final answer from the solution's is A.