Determine standard enthalpy of formation of salt given other reactions I'm a little confused by this question, perhaps my understanding of enthalpy isn't as good as it should be.


  For the compound $\ce{KNO3}$, which of the following reactions is used to determine the standard enthalpy of formation, $\Delta H^\circ_\mathrm f$?
  
  $$\ce{K(s) + N (g) + O3(g) -&gt; KNO3 (s)\\
2K + N2 + 2O3 -&gt; 2KNO3\\
2K + N2 +3O2          -&gt; 2KNO3\\
K  + 1/2 N2 + 3/2 O2  -&gt; KNO3}$$


I know that the $\Delta H^\circ_\mathrm f(\ce{KNO3})= \pu{−494.6 kJ/mol}$ from reading a table, I'm just confused about how do I get from that number to determine the formation of a compound.
The standard enthalpy of formation $\Delta H_\mathrm f$ refers to the formation of 1 mole of product, with the educts and product being in their respective standard states. The fourth equation is therefore correct, as it gives 1 mole $\ce{KNO3}$ as the product. Equation 3 is equation 4 multiplied by 2, which would give $2\Delta H_\mathrm f$. The first two equations can be ruled out because they do not contain the educts nitrogen and/or oxygen in their standard states, which are $\ce{N2}$ and $\ce{O2}$, respectively.