Molar Enthalpy of Combustion of Mg?

Question

Hi, we performed a calorimetric lab in which we did two separate experiments. In the first one we reacted Mg with HCl and in the second one we reacted MgO with HCl. We had to find out their molar enthalpies.
One of the questions are - Explain why and how your calculated enthalpies of reaction would be inaccurate if the surface of the magnesium ribbon had a coating of MgO

Answer 1

If your magnesium ribbon was coated with MgO, then you thought you were determining the enthalpy of reaction of a certain weight of Mg, but really part of that weight was MgO.

So, if you know which has the higher enthalpy of reaction per gram, you can work out whether that would make your answer too high or too low.