Answer:
3) NaCl.
Explanation:
∵ ΔTf = iKf.m
where, i is the van 't Hoff factor.
Kf is the molal depression freezing constant.
m is the molality of the solute.
The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass.
- For most non-electrolytes dissolved in water, the van 't Hoff factor is essentially 1.
So, for sugar: i = 1.
∴ ΔTf for sugar = iKf.m = (1)(Kf)(2.0 m) = 2 Kf.
- For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance.
For NaCl, it is electrolyte compound which dissociates to Na⁺ and Cl⁻.
So, i for NaCl = 2.
∴ ΔTf for NaCl = iKf.m = (2)(Kf)(1.0 m) = 2 Kf.
So, the right choice is: 3) NaCl.
