The question states:2BaCO3(s) ---> 2Ba(s) + 2C(s) + 3O2(g) Change in H = 2432.6kJ First they ask for me to determine the heat of formation of BaCO3. My problem is how do you determine the heat formation of the reactant (in this case) when you are given a change in H for the whole thing. Second they ask how many grams of BaCO3 are formed if 100.0kJ are involved? I have no clue as to which mathematical formula I should use to determine this. Could you please help me to understand this? For your first question, you are correct when you state that you can always calculate the change in H for any reaction, by subtracting the total of the products Hf from the total of the reactants Hf. Change in H = Hf(prod)-Hf(react) In the reaction stated, we know the change in H = 2432.6 kJ.If we set-up the equation just as an algebraic ... The solution uses the relevant theory to explain how to find the heat of formation of a reactant in the reaction given in the question in a clear and concise manner.