Enthalpy problem. HELP ME!!?

Question

I can't figure this out for the life of me. HELP!!

Acetic acid, HC2H3O2, is the sour constituent of vinegar. In an experiment, 3.16 g of acetic acid was burned.

HC2H3O2(l) + 2 O2(g) 2 CO2(g) + 2 H2O(l)

If 45.9 kJ of heat evolved, what is change in H (enthalpy) per mole of acetic acid?

Thanks so much!

Answer 1

HC2H3O2(l) + 2O2(g) ==> 2CO2(g) + 2H2O(l)
When you calculate enthalpy, one very important thing to remember is also to calculate the work done, either by the system or by the environment to the system. However, in this particular case the situation is much simpler, since 2 mole of gas to react and 2 mole of gas to be produced, thus volume change due to the reaction (at the same T and P) is only in the liquid phase and that is negligable. Now, change in H is the same as change in E, the internal energy. Since the molar mass of acetic acid is 60.05 g/mol, we have:
Delta H = -45.9 kJ * 60.05 g/mol /3.16g
= -872 kJ/mol