You divide the actual yield by the theoretical yield and multiply by 100.
EXAMPLE
What is the percent yield if 0.650 g of copper are formed when excess aluminum reacts with a 2.00 g of copper(II) chloride dihydrate according to the equation
Step 1. Write the balanced chemical equation
3CuCl₂·2H₂O + 2Al → 3Cu + 2AlCl₃ + 2H₂O
Step 2. Calculate the theoretical yield of Cu
(a) Calculate the moles of 3CuCl₂·2H₂O
"2.00 g CuCl"_2·2"H"_2"O" × ("1 mol CuCl"_2·2"H"_2"O")/("170.5 g CuCl"_2·2"H"_2"O") = "0.011 73 mol CuCl"_2·2"H"_2"O"2.00 g CuCl2⋅2H2O×1 mol CuCl2⋅2H2O170.5 g CuCl2⋅2H2O=0.011 73 mol CuCl2⋅2H2O