CHEMISTRY
posted by THOMAS .
I know this question is already on here but the answer wasn't helpful to me.
Liquid Sodium can be used as a heat transfer fluid. Its vapor pressure is 40.0 torr at 633 C and 400.0 torr at 823 C. Calculate its heat of vaporization.

I looked and did not see another post of this problem; therefore, I don't know what was confusing you. Wouldn't you use the ClausiusClapeyron equation?

I'm just not sure how to apply it to this problem.

To apply it you simply plug in the numbers, punch them into your calculator, turn the crank and out comes the answers.
ln(p2/p1) = [DHvap/R](1/T1  1/T2)
You have p1 @ T1 and p2 @ T2. R is 8.314, solve for DHvap. The unit for DHvap will be joules. 
100.016 or 1.00 x 10^2 kj/mol