By Alan Sembera – Updated August 30, 2022
When studying the energetics of a chemical reaction, it is often necessary to determine how many moles of a reactant were involved based on the energy change measured in joules. Because a joule quantifies energy and a mole quantifies the amount of substance, converting between the two requires knowledge of the reaction’s enthalpy change per mole (ΔH J mol⁻¹).
For each specific reaction, the amount of energy released or absorbed per mole of substance is a fixed value. For example, the dissolution of sodium hydroxide in water is exothermic and releases approximately 55 200 J per mole of NaOH.
Use calorimetric data or other reliable methods to determine the total joules released or absorbed during the reaction.
Divide the measured energy (in joules) by the reaction’s enthalpy change per mole. If 30 000 J were released, the calculation is 30 000 J ÷ 55 200 J mol⁻¹ ≈ 0.54 mol of NaOH.
