1. 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, (NH 4 ) 2 SO 4. Step 1. Write the formula for the compound. (NH 4 ) 2 SO 4. 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

2. Calculation of percentage composition. Standard Grade Chemistry Calculations for you to try. 1.Calculate the percentage of potassium in potassium phosphate, K 3 PO 4. Formula K 3 PO 4. 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, NH 4 NO 3. Formula NH 4 NO 3. 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. 3. Calculate the percentage of carbon in glucose, C 6 H 12 O 6. Formula C 6 H 12 O 6. 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