[Frederick95]

Given the following information, determine the molar heat of vapourization of water.

H2(g) + 1/2 O2(g) --> H2O(g) + 242.8 kJ
H2(g) + 1/2 O2(g) -->H2O (l) + 286.9 kJ (to 1 decimal place)


Here is my work. What am I doing incorrectly?

242.8 kJ + 286.9 kJ = 529.7 kJ/ 2
                           = 264.85 kJ

Moles = 264.85 kJ / 18.02 g per mol
        =14.7

[DrCMS]

You have two reactions with heats of formation given.  Why do you feel the need to average them and then divide by the molecular weight of water??  What does that give??

What is the difference between the reactions you are given? 
What have you been asked to find?

This is similar to your question about thermite DH and the the way to solve both is similar.  Work out the reaction you have been asked to determine the energy change for and then compare that to the given data.

The question is what the the energy need to do this:
H₂O_(l) -> H₂O_(g)

How do you think you might get that from the reactions you have been given?