How much energy ( in joules) is released when .002 Kg of ethyl alcohol vapor is condensed to a liquid at the same temperature?

Question

 I am unsure of what formula to use and what numbers to plug into it!!

Answer 1

You need to look up the ∆Hvap for ethyl alcohol.  Recall that vaporization and condensation have the same ∆H except for sign.  Vaporization is going from liquid to vapor, and condensation is going from vapor to liquid.  The ∆Hvap for ethyl alcohol is 38.6 kJ/mole and molar mass is 46.1 g/mole.

0.002 Kg x 1000 g/Kg = 2 g 

2 g x 1 mole/46.1 g = 0.043 moles

0.043 moles x -38.6 kJ/mole = -1.67 kJ

-1.67 kJ x 1000 J/kJ = -1670 joules