What is the freezing point of this solution?

Question

A solution is made by mixing 0.265 mol of naphthalene (C10H8, non- ionizing in 1.85 mol of chloroform (CHCl3). What is the freezing point of this solution? For CHCl3. Tf = - 63.5 degree celsius and Kf =-4.68 degree C/m

Answer 1

Hi Russell W.,

Freezing point depression, boiling point elevation, and osmotic pressure are all examples of properties called "colligative properties". That means that you have to calculate them based on a slightly different concentration method than what you are used to, which would be molarity. Instead, you must use molality . That is the number of moles of solute per kilogram of solvent (NOT, per liter!). So go calculate that up, then ratio it appropriately to the given Kf. That gives you the fp depression; add that to the normal freezing point (given in the problem) to get the answer.

Why the different method? Turns out, that this kind of thing only matters according to how many molecules of each thing (solute, solvent) are present. And using kg for the solvent, whatever it is, is just a historical thing -- lots of industrial solvents don't have a fixed composition (look up naphtha, if you don't believe me), so you can't rely on calculations using moles for them, because you don't KNOW the effective molar mass, necessarily. Unless you want to spend all day analysing!

--Cheers, --Mr. d.