1.26 calculate percentage yield

This is a step beyond Section 1.25. We can work out how much product should be made in a reaction, but the yield is rarely 100% for a variety of reasons.

To work out the yield, divide the actual mass made by the theoretical mass and multiply by 100.  (You can also do this by calculating the theoretical moles produced and then dividing the actual moles obtained by the theoretical moles and multiplying by 100.)

e.g. 100 g of ethene is hydrated to ethanol. 110 g of ethanol are made. What is the percentage yield?

C2H4 + H2O → C2H5OH

Moles of C2H4                                    = 100 g ÷ M_r  =  100 ÷ 28                                                 = 3.57 mol

Ratio of C2H4 : C2H5OH             = 1 : 1

Moles of C2H5OH formed           = 3.57 mol (this is the theoretical yield)

Theoretical mass of C2H5OH      = moles x M_r  =  3.57  × 46                                                 = 164 g

% yield                                      = (110 ÷ 164) x 100                                                       = 67 %