# How dependent is the molal depression constant on the nature of solute?

I've been reading about the molal depression constant (freezing point depression constant) for binary solutions in class.

I encountered a problem (see below) in which the constant $$K_f$$ was to be evaluated first for a sucrose-water solution ($$\Delta T$$ and concentration of the solution was given).

Then this value of $$K_f$$ was used to determine the depression $$\Delta T$$ of the freezing point of a glucose-water solution when given it's concentration.

This is the problem:

A $$5\%$$ solution (by mass) of cane sugar in water has freezing point of $$271.00\ K$$. Calculate the freezing point of a $$5\%$$ glucose water if freezing point of pure water is $$273.15\ K$$.

My confusion: Is $$K_f$$ solute dependent? If no, then why not?

I have this confusion because I'm used to solving problems in which if the solute is changed, then most of the constants related to various properties of the solution also changes.

• You're right to be skeptical. This is another case where "ideal behavior" is assumed. More complicated mathematical models can be used to obtain more consistent results. See Wikipedia article Freezing-point depression – MaxW Mar 25 '19 at 16:05

Some solutes form nearly ideal solutions up to moderate molalities, examples being glucose and sucrose in water. Such solutions allow fitting of freezing point temperature data to the following equation:

$$\mathrm{log}(1-x_s)=\frac{\Delta_{fus} H_m}{R}\left(\frac{1}{T_{m}}-\frac{1}{T}\right)$$

Taking various approximations which includes assuming that the heat of fusion is constant from $$T$$ to $$T_m$$ (the melting point of the pure solvent), that $$x_s<<1$$, and that $$T_m\approx T$$ you obtain the expression for the freezing point depression, $$\Delta T = T_m - T$$, in terms of the cryoscopic constant $$K_f$$ and solute molality $$m_s$$:

$$\Delta T = K_f m_s$$

where

$$K_f = \frac{M_wRT_m^2}{\Delta_{fus}H_m}$$

Since $$K_f$$ contains parameters that depend only on the solvent (not on the particular solute) the equation can (to within the limitations imposed by the above approximations) be applied to any solutes with which the solvent forms ideal solutions. Which is why you can determine $$K_f$$ with one solute only to use that same constant to later determine the concentration of another solute.