Question 41

[Guest]

In question 41, I don't understand why the Ka of HF is provided. Were we suppose to use this information to solve the question?

[admin]

Boiling point elevation relates to colligative properties which are properties that depend on the number of particles not the type of particles. The number of particles in this problem is dependent on the concentration times the number of particles present after ionization. Sucrose is one molecule and thus does not ionize. KCL and NaI both can ionize into 2 particles each. Mg(NO3)2 can ionize into 3 particles.

The Ka value of HF is provided to underline the fact that HF is a very weak acid. That means that, for the most part, HF acts almost as one particle since soooooooo little of it ionizes (note that the Ka is very, very low).

[yfangl097543]

There doesn't seem to be a correct answer for 41 listed, so it marks any answer as wrong.

[admin]

The correct answer is B. This has been updated.

[harrington5609]

Why wouldn't Mg(NO3)2 come before NaI? I see that the concentration of NaI is greater, but Mg(NO3)2 dissociates into 3 solute particles where NaI is only 2. Does concentration predominate over the number of ions? They seem to equally affect the fp depression by the equation delta T = Kf x m x i

[harrington5609]

my apologies. i just read the question over and its bp elevation not fp depression. Same concept but it made my last post sound crazy

[curlybrand4149]

I understand that the more molecules it is dissolved to, the higher the bp elevation. But isn't change in bp dependent on molality and not molarity? Is there a way to convert them - and if so, why don't we?

[mcat_premed3832]

molality = moles solute per kg solvent

the solvent is water

so molality = moles solute per 1000 g water

but the density of water is 1 g/ml or 1000 g/ 1000 ml or 1000 g per liter (at 25 oC)

so for water in all but extreme circumstances (high concentrations, non MCAT temps/conditions!) . . .

molarity and molality are very good approximations.


http://chemistry.about.com/od/lecturenotesl3/a/concentration.htm