You mix 125 mL of 0.180 M with 50.0 mL of 0.450 M in a coffee-cup calorimeter, and the temperature of both solutions rises from 21.80 °C b?

No. of moles of the solute = (0.180 mol/L) × (125/1000 L) + (0.450 mol/L) × (50.0/1000 L) = 0.045 mol

Volume of the final solution = (125 + 50.0) mL = 175 mL
Mass of the final solution = (175 mL) × (1.00 g/mL) = 175 g
Energy released = m c ΔT = (175 g) × (4.2 J/g K) × (23.88 - 21.80 K) = 1530 J

Enthalpy of reaction per mole of solute = -(1530 J) / (0.045 mol) = -34000 J/mol = -34 kJ/mol

(The "-" sign indicates the change is exothermic as the temperature of the solution rises.)