This is how you should be doing,
P reacts with excess Cl₂
2P + 3Cl₂ ----> 2PCl₃
Cl₂ is excess therefore amount of PCl₃ formed will depend on P.
According to stoichiometry,
2 moles of P gives = 2 moles of PCl₃
∴ 5 moles of P will give = 5*2/2 = 5 moles of PCl₃
You are correct.
