I calculated the standard enthalpy of a reaction using the constituent standard enthalpies of formations. My answer is in kcal / mol, how do I convert to J/g ? I know how to do the unit analysis BUT for the weight in grams... I imagine I use the weight in grams of the reaction - right? Would that be the combined weight in grams of the products or the net difference (in grams?)? ------ example, calculating the enthapy of the following reaction: CuSO4•5H2O(s) → CuSO4•3H2O(s) + 2H2O(g) literature values in kcal/mol : CuSO4•5H2O(s) ΔHf = -544.45 CuSO4•3H2O(s) ΔHf = -402.27 H2O(g) ΔHf = -57.80 When I convert each one to J/g, I get an enormous enthalpy of reaction (about -26000) but it should be about +500!!!!!!!!!