Okay so I think I know what I'm doing, but I have a lab tommorow and I have to use Hess's Law to find the total enthalpy change of Mg(s)+1/2O2(g)-->MgO(s). I know how to do that but during my lab tommorow I have to find the enthalpy change of 2 reactions. First, Mg(S)+2HCL(aq)-->H2(g)+MgCl2(aq), using 0.5g of magnesium and 100.0 ml of 1.00 mol/L. So, I should record the temperature change of the original HCl, then the temperature of the MgCl2 afterwords and record the temperature change (this is a high school lab, so it doesn't matter that some energy is lost to surroundings). I understand all the formulas but I'm confused which one is the limiting reagent, which would influence which one I use in the q=mcΔT and which value I would use for the amount of moles. So, from what I have so far, I have ΔH(sol)=(0.5g+100.0g)(4.18 J/g°C)(ΔT)/the moles of the Mg, which is 0.021 (assuming the specific heat capacity of HCl is 4.18 J/g°c) Also, same question for the nest equation, MgO(s)+2HCl(aq)-->H2O(l) + MgCl2(aq)... which value should I use for m and which for the n? Sorry for the long question, but thanks for any input. I know the text is long, but to sum it up, how do you decide in a chemical reaction which reactant to use in the molar enthalpy equations to find the correct molar enthalpy? Thank you.