Question

Assertion: The depression of freezing point of 0.1 molal aqueous solution of MgCl₂ is less than that of 0.1 molal aqueous solution of NaCl.

Reason: The number of particles furnished by 0.1 molal MgCl₂ is less than that of 0.1 molal NaCl.

Options

• Both Assertion and Reason are true, and Reason is the correct explanation for Assertion.

• Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion.

• Assertion is true and Reason is false.

• Both Assertion and Reason are false.

Answer 1

Both Assertion and Reason are false.

Explanation:

The depression of freezing point (ΔT_f) is a colligative property, meaning it depends on the number of particles in the solution. This is calculated using the formula: 

ΔT_f = i × K_f × m

(Where i is the van’t Hoff factor, K_f the cryoscopic constant, and m is molality.)

For NaCl: It dissociates into 2 particles.

\[\ce{NaCl → Na+ + Cl−}\]

For MgCl₂: It dissociates into 3 particles.

\[\ce{MgCl2 → Mg^2+ + 2Cl−}\]

Since MgCl₂ produces more particles (i = 3) than NaCl (i = 2), the depression of freezing point MgCl₂ will be greater, not less.

For the same molality (0.1 m), MgCl2 therefore produces more particles (0.3 moles/kg) than 0.1 molal NaCl (0.2 moles/kg) and so gives a larger ΔT_f, not a smaller one.