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:
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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 %
