A 0.158 g sample of magnesium metal reacts completely with 100.0 mL of 1.0 M hydrochloric acid in a coffee cup calorimeter. The temperature of the solution rose from 25.6°C to 32.8°C. What is ∆Hrxn? Assume the specific heat of the solution is 4.184 J/mol-K and the density is 1.0 g/mL.

Answer 1

Mg + 2HCl ==> MgCl2 + H2

q = mass H2O x specific heat solution x (Tfinal-Tinitial)
q is delta H. This will give you delta H for the reaction. Most of these problems, however, want delta H in kJ/mol and not just joules; i.e., they don't state the problem correctly and assume you know they want it in kJ/mol and not just kJ for the reaction in the problem.
(q/0.158)*atomic mass Mg will give you J/mol and you can convert that to kJ/mol.