Question

Compound: Concentration: Measured deltaT
LiCl 6.00 g/kg 0.492 deg C
HCl 4.00 g/kg 0.389 deg C
NaCl 6.00 g/kg 0.356 deg C

Determine the value of the vant hoff factor for each salt for the experimentally measured delta T value. (kf=1.86)

I have tried to figure this out, but it is saying I'm wrong. I might be doing the math wrong. Can you help me?

Answer 1

dT = i*Kf*m
i = dT/(Kf*m)
dT = 0.492 for LiCl
Kf = 1.86 (I assume you are using this value or one given in class to use.
m = mols/kg solvent
mols = 6.00/molar mass LiCl
I get 1.869 for i which would round to 1.87.
If you post your work for one of the problems I will find your error for you.

Answer 2

I calculate the amount of kg, which is .00600 kg. Then I calculate the molality:

.14 mols/.00600kg = 23.3 m
23.3*1.86=43.338
.492/43.338=.0114

I don't know which step is incorrect.

Answer 3

The way I read the LiCl part of the problem, (LiCl) = 6.00 g/kg ; therefore, mols LiCl = 6.00 g/42.394 = 0.1415
m = 0.1415 mols/1 kg solvent.
i = 0.492/(0.1415*1.86) = 1.869 which I rounded to 1.87. Therefore, i isn't quite 2 as we would predict for two particles (Li^+ and Cl^-) but just less than that. Your error is in calling the solvent 0.006 kg; the problem says it is 1 kg (at least 6.00 g LiCl/kg says that to me). That means to me you would have 12.00 g/2 kg, 18.00 g/3 kg etc.

Answer 4

thank you! I figured that was the problem.