A scientist measures the standard enthalpy change for the following reaction to be -172.4 kJ :

Question

2SO2(g) + O2(g) 2SO3(g)

Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of SO2(g) is what in kJ/mol?

Answer 1

dHrxn = (n*dHf products) - (n*dHf reactants)
You know dHrxn and you can look up dHf O2 (it is zero) and SO3, calculate the only unknown of dHf SO2.