First of all understand the word yield in terms of chemistry.Yield is actually the mass of product formed in a chemical reaction.
Percent yield is a formula which is ;
Percentage Yield = mass of Actual Yield x 100%
mass of Theoretical Yield
Now, you need to know about two more defination here actual yield and theoretical yield
Actual yield is the mass of product in a reaction experimentally or duing industrial procedure
Theoretical yield is the mass of product that we predict or calculate using balanced chemical equation.
Conclusion : Percent yield describes how much of a material was produced in a reaction compared to
how much would be produced in an ideal reaction with no losses.
See the example below;
Sample problem :112g of nitrogen gas reacts with hydrogen gas to produce 40.8g of ammonia gas according to the equation given below:
N2(g) + 3H2(g) 2NH3(g)
Calculate the percentage yield of ammonia.
Solution : Actual yield of ammonia (NH3) is 40.8g
Theoretical yield of ammonia (NH3) is calculated using the balanced equation:
From the balanced chemical equation we can see that the mole ratio of N2 and NH3 is 1:2
convert mass of N2 to mole
moles N2 = 112 / 28 =4 moles of N2
4 moles of N2 x 2 moles of NH3 / 1 mole of N2 = 8 moles of NH3
now, convert moles of NH3 to mass and this mass would be the theoretical yield of NH3.
theoretical yield NH3 = predicted mass NH3
predicted mass NH3 = moles NH3 x molar mass NH3 = 8 x 17 = 136g
Percentage yield = (actual yield ÷ theoretical yield) x 100
percentage yield NH3 = (40.8 ÷ 136) x 100 = 30%