Greetings all. I had a lab from which I measured 6822.8 joules released by the reaction of Mg+2HCl->MgCl2+H2. The limiting reagent is Mg. I used 1.68E-2 moles (.0168 moles) of Mg meaning there is the same number of moles of MgCl2. Dividing 6822.8 Joules by .0168 to get joules per mole (and kj per mol) gives -406.1kj/mol.
Problem is is that apparently the heat of formation of MgCl2 is -641.8kj/mol.
Any suggestion as to where I went wrong? thanks