HIGHER GRADE CHEMISTRY CALCULATIONS

Calculation of percentage yield.

Percentage yield is calculated using the formula:-

percentage yield = x 100

The actual yield is given in the question. The theoretical yield is calculated from the balanced equation.

actual yieldtheoretical yield

Worked example.

In a reaction 320 kg of sulphur dioxide reacted with oxygen to form 360kg of sulphur trioxide. Calculate the percentage yield.

Find the theoretical yield using the balanced equation:-

2SO2 + O2 2SO3

2 mol 2 mol

128.2 g 160.2 g

128.2 kg 160.2 kg

320 kg 399.9 kg

percentage yield = x 100 = 90.0%

360

399.9

Higher Grade Chemistry

Calculation of percentage yield.

Calculations for you to try.

1. 0.5 tonnes of hydrogen reacts with excess nitrogen to form

0.66 tonnes of ammonia.

N2 + 3H2 2NH3

Calculate the percentage yield of ammonia.

N2 + 3H2 2NH3

3 mol 2 mol

6 g 34 g

6 tonnes 34 tonnes

0.5 tonnes 2.83 tonnes

percentage yield = x 100 = 23.32%

0.66

2.83

Higher Grade Chemistry

Calculation of percentage yield.

Calculations for you to try.

2. Ethene reacts with steam to ethanol

C2H4 + H2O C2H5OH

Calculate the percentage yield of ethanol when 140g of

ethene reacts to form 170 g of ethanol.

C2H4 + H2O C2H5OH

1 mol 1 mol

28 g 46 g

140 g 140/28 x 46 g

230 g

percentage yield = x 100 = 73.9%

170

230