Question

2.5 moles of Cl2 reacts with a large excess of P4. The actual yield of PCl5 is 0.50 moles. What is the percent yield?

P4(s) + 10 Cl2 (g)-> 4 PCl5 (g)

Answer 1

10Cl2 + P4 ==> 4PCl5

Convert 2.5 mol Cl2 to mols PCl5 using the coefficients in the balanced equation. That is the theoretical yield of the reaction.
Then % yield = [theoretical yield/actual yield]*100.
The actual yield is given in the problem as 0.5 mol.