1. Chapter 9.5 Percentage Composition by Mass

2. Percentage Composition of Compounds Chemists need to conduct analysis in order to find out how much of each element there is in a new compound. They do so by finding out the mass of each element in the compound. In this way, chemists know the percentage composition of a compound .

3. How do we find the percentage composition of element? Percentage by mass of an element in a compound A r of element × number of atoms in formula M r of compound =  100% The percentage by mass of an element in a compound can be found using the formula:

6. Worked Example 2: Hydrogen Peroxide (H 2 O 2 ) M r of hydrogen peroxide (H 2 O 2 ) = (2 × 1) + (2 × 16) = 34 Percentage of hydrogen in hydrogen peroxide A r of hydrogen × number of hydrogen atoms M r of hydrogen peroxide = 1  2 34 =  100%  100% Calculate the percentage composition by mass of hydrogen and oxygen present in hydrogen peroxide (H 2 O 2 ) = 5.9 %

7. Percentage of oxygen in hydrogen peroxide A r of oxygen × number of oxygen atoms M r of hydrogen peroxide = 16  2 34 =  100%  100% = 94.1 %

8. Example 3 Calculate the percentage of water in copper(II) sulphate crystals (CuSO 4 .5H 2 O). M r of copper(II) sulphate crystals = 64 + 32 + (4 × 16) + (5 × 18) = 250 M r of water = (1 × 2) + 16 = 18 Percentage of water in CuSO 4 .5H 2 O M r of water × number of water molecules M r of copper(II) sulphate crystals = 18  5 250 =  100%  100% = 36 %