STANDARD GRADE CHEMISTRY CALCULATIONS

Calculation of percentage composition.

Percentage composition allows us to calculate the percentage of any element present in a compound.

Worked example.

Calculate the percentage of nitrogen in ammonium sulphate, (NH4)

2SO4.

Step 1. Write the formula for the compound. (NH4) 2SO4.

Step 2. Calculate the formula mass. (2 x 14) +(8 X 1) + (1 x 32) + (4 x16) = 132

Step 3. Work out the mass of the required element (nitrogen) = 28

Step 4. Calculate the percentage of the element x 100

= 21.2%

28

132

Calculation of percentage composition.

Standard Grade Chemistry

Calculations for you to try.

1. Calculate the percentage of potassium in potassium phosphate, K3PO4.

Formula K3PO4.

Formula mass (3 x 39) + (1 x 31) + (4 x 16) = 212

Mass of phosphorus in compound (3 x 39) = 117

% of phosphorus in compound = x 100 = 55.19%

117

212

2. Calculate the percentage of nitrogen in ammonium nitrate, NH4NO3.

Formula NH4NO3.

Formula mass (2 x 14) + (4 x 1) + (3 x 16) = 80.

Mass of nitrogen in compound (2 x 14) = 28

% of nitrogen in compound = x 100 = 35%28

80

3. Calculate the percentage of carbon in glucose, C6H12O6.

Formula C6H12O6.

Formula mass (6 x 12) + (12 x 1) + (6 x 16) = 180.

Mass of carbon in compound (6 x 12) = 72

% of carbon in compound = x 100 = 40%71

180

Standard Grade Chemistry