The enthalpy of reaction is already given to be -2221 kJ per mole of propane reacted. So, first let's determine the number of moles of propane. The molar mass of propane is 44 g/mol.
400 g C₃H₈ (1 mol/44g) = 9.091 moles
Thus, the enthalpy change is
ΔH = -2,221 kJ/mol * 9.091 mol = -20,191.11 kJ