[ajschaller]

Thank you for even reading this.
I dont think its that changelling i just dont understand it for some weird reason...right here's the question :

Question -   Nitric acid can be made by reacting water, nitrogen dioxide and oxygen according to
the following equation.

H2O(l) + 2NO2(g) + O2(g) → 2HNO3(l) ΔH = –128 kJ mol–1
Some standard enthalpies of formation, ΔHf , are givenbelow.
 
ΔHf / kJ mol–1 of Substance: H2O(l) is -286           NO2(g) is + 34            O2(g) is 0 (because its an element)


Use the data above to calculate a value for the standard enthalpy of formation of
nitric acid.




Right thats the question. Now Ive seen the answer and tried to work it out from there but still could not understand, so if anyone does answer this id be gratful if you would spell it out step by stelp slowly because ive already tried quite hard to do it.  This is the last thing in this module I need to know and exams on the 9th.Thank you very much in advance.

[Astrokel]

First, write out the chemical equation of standard enthalpy of formation of nitric acid and on top of that you have  :delta: H_f of water and nitrogen dioxide. Now you just have to find a way to link all this informations up using energy cycle or hess' law.

Well if it is unclear to you of what i said, i will explain it again, let me know.