[A5HLEY]

Ok, so I'm working on some review problems for my semester exam, and I'm stuck on this one.

A solution is prepared by dissolving 2.30g of an unknown nonelectrolyte in 16.00g of cylcohexane (tf=6.6 degrees celsius, and kf=20.0 degreens celsius/m). What is the molar mass of the unknown compound if the solution has a freezing point of -4.3 degrees celsius?

I'm thinking that you use the forumla (Change in Tf) = im(kf), but I'm not really sure where to go from there.

Any suggestions would be appreciated!

[ARGOS++]

Dear A5HLEY;

Why not search on the Forum?, - Same and similar problems are already solved several times.

On example is:   "Freezing point depression and molecular weight”

Good Luck!
                   ARGOS⁺⁺

[A5HLEY]

Dear A5HLEY;

Why not search on the Forum?, - Same and similar problems are already solved several times.

On example is:   "Freezing point depression and molecular weight”

Good Luck!
                   ARGOS⁺⁺

Ok, so I pretty much understand that example and I know how to apply it to my problem, except for one thing. How do I figure out the molality for the unknown solute? Isn't molality (mol solute/kg solvent)? If so, how do I find convert the grams of unknown to moles?

I think I'm missing something...

[ARGOS++]

Dear A5HLEY;

It’s much simpler in your case, as MolaLity is required, and that is mole/kg SoluTION!

MolaRity is contrary: mole/Liter SoluTION!
So I think you can go “full steam” ahead.

Good Luck!
                   ARGOS⁺⁺

P.S.:  Sorry for my mistake!
.

[A5HLEY]

Oh wait, I didn't see your edit. Let me try again.

[A5HLEY]

Yikes!! I still think I'm doing something incorrectly, as I got a rather large answer.

Ok, so first I converted the 16 g of cyclohexane to mol:

(16 g)(1 mol)/(84.16g) = .1901140684 mol cyclohexane

Then I found m:

(.1901140684 mol)/(.016 kg) = .0030418251

So I found that MW = (2.30)/(.0030418251) = 756.1249988 g/mol

The value is too large, I think!

[ARGOS++]

Dear A5HLEY;

It seems you have misunderstood the given Example!
(You don’t need any Moles of Cyclohexane!)

First simple question:     How much is your measured Δt ?
Second:  What is the MolaLity of your Solution by using the formula?
Third:     What is the Concentration of the Solution in g/Kg?
And finally: What is the MW if you compare Second and Third?

Could this by a little simpler? (Make you a picture!)

Good Luck!
                   ARGOS⁺⁺