What equation do you use to calculate the molar enthalpy of reaction when you are only given the mass of the substance, and the initial and final temperatures?

Mass of Mg is 0.31g
Initial temp in calorimeter 24.1
Final temp in calorimeter 36.8
and 100 mL of HCl soultion

I'm stumped, please help!

Okay.

You need to first find the amount of heat evolved during the reaction, using the formula:

Q = mc\Delta T

Delta T is the change in temperature,
m is the mass of substance which experienced the change in temperature,
c is the specific heat capacity of that substance,
Q is the amount of heat involved.

Delta T = 36.8 - 24.1 = 12.7 K
c is approximately 4.3 J/(gK)
m is 100 g (100 mL has a mass of approximately 100 g)

So, you can find the heat evolved.

How many moles of Mg was used? You need to find the heat that 1 mole of Mg would have produced.

0.31 g of Mg contains 0.31/24 = 0.0129 mol of Mg

So, 0.0129 mol released the amount of energy you just calculated.

How much energy does 1 mole of Mg release? This is your molar enthalpy of reaction.

I hope it helped! :)