How to find ?G°rxn when given only some of the ?G°f amounts?

Question

I am not sure how to calculate ?G°rxn if I am not given all of the values for ?G°f. Can someone explain? I thought you

 

 

The question I am trying to answer --

 

Calculate the ?G°rxn using the following information.

                     4 HNO3(g) + 5 N2H4(l) →7 N2(g) + 12 H2O(l) ?G°rxn = ?

 

?G°f (kJ/mol)      -73.5          149.3                       -237.1

 

How do I find the missing value to calculate the products-reactants equation?

Answer 1

N2(g) is the considered the standard state of nitrogen, so it has an energy of formation of zero.

 

Does that help? :)