You wish to know the enthalpy change for the formation of liquid from PCl3 from the elements.
P4(s) + 6 Cl2(g) ? 4 PCl3(l) ?rH° = ?
The enthalpy change for the formation of PCl5 from the elements can be determined experimentally, as can the enthalpy for the reaction of PCl3(l) with more chlorine to give PCl5(s):
P4(s) + 10 Cl2(g) ? 4 PCl5(s) ?rH° = -1774.0 kJ/mol-rxn
PCl3(l) + Cl2(g) ? PCl5(s) ?rH° = -123.8 kJ/mol-rxn
Use these data to calculate the enthalpy change for the formation of 1.00 mol of PCl3(l) from phosphorous and chlorine.