1. The problem statement, all variables and given/known data ok so i'm trying to find the change in enthalpy for this reaction: Mg(s) + (1/2)O2--> MgO(s) I've done the lab to find the temperatures of certain reactions, these were: 9.70 Celsius for the reaction Mg(s) + 2HCl(aq)--> MgCl2(aq) + H2(l) 7.60 Celsius for the reaction MgO(s) + 2HCl --> MgCl2(aq) + H2O(l) reaction (and we know the enthalpy of change for the reaction H2(g) + (1/2)O2(g) --> H2O(l) is 285.8 kJ) 2. Relevant equations change in enthalpy = mass (100ml/1000ml) * specific heat (4.18) * change in temp (above) 3. The attempt at a solution using the (above) equation i found that the enthalpy of change for the first reaction was -405 kJ per mol and the second was -318 kJ per mol. so i built a Hess's law chart and found that [Mg(s) + 2HCl(aq)--> MgCl2(aq) + H2(l)] + [H2(g) + (1/2)O2(g) --> H2] = [MgO(s) + 2HCl --> MgCl2(aq) + H2O(l)] + [Mg(s) + (1/2)O2--> MgO(s)] so basically the equation look like this: -405 + (-285.8) = -318 + x therefore x should be -372.8 kJ per mol but all the answers i've found on the net so far say that the enthalpy of combustion is approximately 602 kJ per mol help?