Solving moles-to-moles limiting reactant problems

Question

Solid phosphorus (P) and chlorine (Cl₂) gas react to form solid phosphorus pentachloride (PCl₅). Suppose you have 2.0 mol of P and 1.0 mol of Cl₂ in a reactor.

Suppose as much as possible of the P reacts. How much will be left? Round your answer to the nearest 0.1 mol.

Answer 1

2P(s) + 5Cl₂(g) ==> 2PCl₅(s) ... balanced equation

2 mol......1 mol....

Since the mole ratio of P : Cl2 tells us we need 5 mols Cl2 for every 2 mols P, it is clear that Cl2 is limiting and will run out before the P does.

How much P will react?

1 mol Cl2 x 2 mol P / 5 mol Cl2 = 0.40 moles of P will react

How much P will be left?

2.0 mol - 0.40 mol = 1.6 mol of P will be left