ΔHrx of magnesium in hydrochloric acid?

Let's do the sign first. I actually hate these signs, because they can be positive or negative depending on how you define everything else. Your ΔT is probably defined as the final temp minus the initial temp. BTW I think you meant −(heat capacity of water x m of water x Cp ) instead of adding the Cp. Now if we calculate the ΔHrx with ΔT as a positive number, without the negative sign in front, the heat would come out as a positive number. According to the rules of thermo, a positive ΔH would mean that heat is going into the system. What we observed is that heat is coming out of the system (it got hot), so that has to be a negative. Now for the rest of it, the heat of the reaction goes into the water in the system and makes it rise. The amount of heat and its relationship to the temperature is q = m X Cp x ΔT so the heat coming out of the reaction (negative) goes into the water and raises its temperature. This heat you observe is for your amount (in moles) of magnesium that you use. You would then divide the answer you get for ΔHrx by the moles of magnesium that you used to get the ΔHrx per mole.