[Acailiya]

There's this lab where I'm reacting it with 100mL of HCl (aq) and it has two portions to it.

Part A (results):
Mass of Mg(s) 0.31 g
Initial temperature of calorimeter contents 24.1 ºC
Final temperature of calorimeter contents 36.8 ºC

Part B (results):
Mass of MgO(s) 1.22 g
Initial temperature of calorimeter contents 24.0 ºC
Final temperature of calorimeter contents 31.9 ºC

calculate the molar enthalpies of reaction for the reactions occurring in Parts A and B

[mikasaur]

Hello and welcome to the forum! Before we can help you you must show that you've made an attempt to solve the questions. What have you done so far?

[Acailiya]

Q=mcΔt
  =(1.22g) (4.19j/g°c)(31.9-24.0)
  = 3.01261 kj >3.0 kj

=       3.01261
 (1.22)x (1mol/40.31g)

= 0.099> -0.10mj/mol

Part A, I completed this way as well. It just seems off to me. Part A I got -0.41 MJ/Mol

[mikasaur]

I'm not sure where your value of 3.01261 kJ is coming from. Or where the negative sign comes in...

[Enthalpy]

I disagree with the mass used to compute the heat capacity and the heat. We don't have the full description here, but if reacting 100mL aqueous HCl, the products probably include much water which is heated as well. By the way, just 10K heating indicates a significant heat capacity in the calorimeter.

I'm pleased with a negative enthalpy of reaction when heat is produced.

May I suggest to watch the capital letters at the units? Big J and M for joule and mega, small m for milli.

I'd like to recommend too to learn a few figures, not just formulas, so you can double-check the credibility of your results. Like: water's heat capacity, enthalpy of melting, enthalpy of vaporization, the heat of formation of CO₂ and H₂O, things like that. Not just for thermochemistry: everywhere.