Question

In comparison to a 0.01 M solution of glucose, the depression in freezing point of a 0.01 M MgCl₂ solution is ________ .

• A. About six times
• B. About twice
• C. About three times
• D. The same

Answer 1

The Correct Option is C

The depression in freezing point (ΔTf) of a solution is given by the formula:

ΔTf = i * Kf * m

where:

• i is the van't Hoff factor
• Kf is the molal freezing point depression constant (same for both solutions here)
• m is the molality of the solution

For nonelectrolyte solutions like glucose, which do not dissociate in water, the van’t Hoff factor i = 1. Thus, for a 0.01 M glucose solution:

ΔTf (glucose) = 1 * Kf * 0.01

MgCl₂, however, dissociates into three ions in solution (Mg²⁺ and 2 Cl^-), so the van’t Hoff factor i = 3. For a 0.01 M MgCl₂ solution:

ΔTf (MgCl₂) = 3 * Kf * 0.01

Thus, the depression in freezing point for MgCl₂ is three times that of the glucose solution:

ΔTf (MgCl₂) = 3 * ΔTf (glucose)

Therefore, the correct answer is: About three times.