Question

1.24 g of \(AX_2\) (molar mass 124 g mol\(^{-1}\)) is dissolved in 1 kg of water to form a solution with boiling point of 100.105°C, while 2.54 g of AY_2 (molar mass 250 g mol\(^{-1}\)) in 2 kg of water constitutes a solution with a boiling point of 100.026°C. \(Kb(H)_2\)\(\text(O)\) = 0.52 K kg mol\(^{-1}\). Which of the following is correct?

• A. AX₂ is completely unionised while AY₂ is fully ionised.
• B. AX₂ is fully ionised while AY₂ is completely unionised.
• C. AX₂ and AY₂ (both) are completely unionised.
• D. AX₂ and AY₂ (both) are fully ionised.

Hint:

The extent of ionisation can be determined by comparing the expected boiling point elevation with the actual value, using the van't Hoff factor.

Answer 1

The Correct Option is A

This problem is solved using boiling point elevation. The relevant formula is:

\[\Delta T_b = i \cdot K_b \cdot m\]

Here, \(\Delta T_b\) denotes the increase in boiling point, \(i\) is the van 't Hoff factor (ionization number), \(K_b\) is the ebullioscopic constant, and \(m\) represents the molality of the solution.

Step 1: Calculate Molality for Each Solution

For AX₂:

Mass dissolved \(= 1.24 \, \text{g}\)

Molar mass \(= 124 \, \text{g/mol}\)

Molality (\(m\)) is calculated as follows:

\[m = \frac{\text{mass of solute (g)}}{\text{molar mass (g/mol)}} \times \frac{1}{\text{mass of solvent (kg)}} = \frac{1.24 \, \text{g}}{124 \, \text{g/mol}} \times \frac{1}{1 \, \text{kg}} = 0.01 \, \text{mol/kg}\]

For AY₂:

Mass dissolved \(= 2.54 \, \text{g}\)

Molar mass \(= 250 \, \text{g/mol}\)

The molality (\(m\)) is:

\[m = \frac{2.54 \, \text{g}}{250 \, \text{g/mol}} \times \frac{1}{2 \, \text{kg}} = 0.00508 \, \text{mol/kg}\]

Step 2: Determine the van 't Hoff Factor (i)

Given \(K_b = 0.52 \, \text{K kg/mol}\).

For AX₂:

Boiling point change \(\Delta T_b = 100.105 - 100 = 0.105 \, °\text{C}\)

\(i \cdot 0.52 \cdot 0.01 = 0.105\)

\[i = \frac{0.105}{0.52 \times 0.01} = 20.192\]

An \(i\) value of \(1\) indicates no ionization; thus, AX₂ is unionized.

For AY₂:

Boiling point change \(\Delta T_b = 100.026 - 100 = 0.026 \, °\text{C}\)

\(i \cdot 0.52 \cdot 0.00508 = 0.026\)

\[i = \frac{0.026}{0.52 \times 0.00508} \approx 0.985\]

This \(i \approx 2\) suggests AY₂ is ionized, dissociating into AY and 2Y⁻ (fully ionized).

Conclusion

The calculations demonstrate that AX₂ is completely unionized, while AY₂ is fully ionized, confirming the assertion: "AX₂ is completely unionised while AY₂ is fully ionised."