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# The Mole 9.5

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### The Mole 9.5

1. 1. Chapter 9.5 Percentage Composition by Mass
2. 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. 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:
4. 4. Worked Example 1: Ammonia (NH 3 ) <ul><li>Calculate the percentage composition by mass of nitrogen and hydrogen present in ammonia, NH 3 . </li></ul><ul><li>Step 1: </li></ul><ul><li>Relative molecular mass (M r ) of ammonia </li></ul><ul><li>= 1 x A r of N + 3 x A r of H </li></ul><ul><li>= 14 + 3 </li></ul><ul><li>= 17 </li></ul>
5. 5. <ul><li>Percentage composition of nitrogen in ammonia </li></ul><ul><li>= A r of nitrogen x number of nitrogen atoms in formula X 100% </li></ul><ul><li> relative molecular mass (M r ) of ammonia </li></ul><ul><li>= 14 x1 x 100% 17 </li></ul><ul><li>= 82.4 % </li></ul><ul><li>Percentage composition of hydrogen in a ammonia = A r of hydrogen x number of atoms in formula X 100% </li></ul><ul><li>relative molecular mass (M r ) of ammonia </li></ul><ul><li>= 1 x 3 x 100% </li></ul><ul><li>17 </li></ul><ul><li>= 17.6 % </li></ul><ul><li>  </li></ul>
6. 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. 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. 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 %
9. 9. Worked Example 4 <ul><li>Calculate the mass of calcium in 25g of calcium nitrate Ca(NO 3 ) 2 </li></ul><ul><li>Step 1: Mr of Ca(NO 3 ) 2 </li></ul><ul><li>= 40 + 2 x (14 + 16 x 3) = 164 </li></ul>
10. 10. <ul><li>Step 2: Percentage composition by mass of Ca in Ca(NO 3 ) 2 </li></ul><ul><li>= A r of Ca x number of Ca atoms in formula X 100% </li></ul><ul><li>M r of Ca(NO 3 ) 2 </li></ul><ul><li>= 40 X 1 X 100% </li></ul><ul><li>164 </li></ul><ul><li>= 24.39% </li></ul>
11. 11. <ul><li>Step 3: </li></ul><ul><li>Mass of calcium in 25g of calcium nitrate </li></ul><ul><li>= 24.39/100 X 25g = 6.10g </li></ul>
12. 12. Worked Example 5 <ul><li>Calculate the mass of water in 54.1g of FeCl 3 . 6H 2 O </li></ul><ul><li>Step 1: Mr of FeCl 3 . 6H 2 O </li></ul><ul><li>= 56 + 35.5 X 3 + 6 x (1 X 2 + 16) = 270.5 </li></ul><ul><li>Step 2: Mr of water, H 2 O = 2 X 1 + 16 = 18 </li></ul><ul><li>  </li></ul>
13. 13. <ul><li>Step 3: Percentage composition by mass of water in FeCl 3 . 6H 2 O </li></ul><ul><li>= M r of water x No. of water molecules in formula X 100% </li></ul><ul><li>M r of FeCl 3 . 6H 2 O </li></ul><ul><li>= 18 X 6 X 100% </li></ul><ul><li>270.5 </li></ul><ul><li>= 39.9% </li></ul><ul><li>Step 4: Mass of water in 54.1g of FeCl 3 . 6H 2 O </li></ul><ul><li>= 39.9/ 100 X 54.1 = 21.6g </li></ul>Worked Example 5 (cont’d)