2CH2O2 + O2 --> 2CO2 + 2H2O
The ∆H for the reaction is based on the moles of reactants and products in the balanced equation. So, in this case, it will be the heat associated with the combustion of TWO moles of CH2O2. We are given the heat associated with combustion of 0.152 moles. So, it is a simple matter to do the conversion.
38.7 kJ/0.152 moles CH2O2 x 2 mol CH2O2 = 509 kJ = ∆Hrxn