the mole 9.5

Chapter 9.5Percentage Composition

by Mass

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.

How do we find the percentage composition of

element?

Percentage by mass of an element in a compound

Ar of element × number of atoms in formula

Mr of compound

= 100%

The percentage by mass of an element in a compound can be found using the formula:

Worked Example 1:Ammonia (NH3)

• Calculate the percentage composition by mass of nitrogen and hydrogen present in ammonia, NH3.

• Step 1:Relative molecular mass (Mr) of ammonia

= 1 x A r of N + 3 x A r of H

= 14 + 3= 17

• Percentage composition of nitrogen in ammonia = A r of nitrogen x number of nitrogen atoms in formula X 100%

relative molecular mass (M r) of ammonia

= 14 x1 x 100% 17= 82.4 %

• Percentage composition of hydrogen in a ammonia = A r of hydrogen x number of atoms in formula X 100%

• relative molecular mass (M r) of ammonia

• = 1 x 3 x 100%

• 17• = 17.6 %•  

Worked Example 2: Hydrogen Peroxide (H2O2)

Mr of hydrogen peroxide (H2O2) = (2 × 1) + (2 × 16) = 34

Percentage of hydrogen in hydrogen peroxide

Ar of hydrogen × number of hydrogen atoms

Mr of hydrogen peroxide =

1 2

34=

= 5.9 %

100%

100%

Calculate the percentage composition by mass of hydrogen and oxygen present in hydrogen peroxide (H2O2)

Percentage of oxygen in hydrogen peroxide

Ar of oxygen × number of oxygen atoms

Mr of hydrogen peroxide =

16 2

34=

= 94.1 %

100%

100%

Example 3 Calculate the percentage of water in copper(II) sulphate crystals (CuSO4.5H2O).

Mr of copper(II) sulphate crystals = 64 + 32 + (4 × 16) + (5 × 18) = 250

Mr of water = (1 × 2) + 16 = 18

Percentage of water in CuSO4.5H2OMr of water × number of water molecules

Mr of copper(II) sulphate crystals =

18 5

250=

= 36 %

100%

100%

Worked Example 4

• Calculate the mass of calcium in 25g of calcium nitrate Ca(NO3)2

• Step 1: Mr of Ca(NO3)2

= 40 + 2 x (14 + 16 x 3) = 164

• Step 2: Percentage composition by mass of Ca in Ca(NO3)2

= A r of Ca x number of Ca atoms in formula X 100%

M r of Ca(NO3)2

= 40 X 1 X 100% 164

= 24.39%

• Step 3: Mass of calcium in 25g of calcium nitrate = 24.39/100 X 25g= 6.10g

Worked Example 5

Calculate the mass of water in 54.1g of FeCl3. 6H2O

Step 1: Mr of FeCl3. 6H2O

= 56 + 35.5 X 3 + 6 x (1 X 2 + 16) = 270.5

Step 2: Mr of water, H2O = 2 X 1 + 16 = 18

• Step 3: Percentage composition by mass of water in FeCl3. 6H2O

= Mr of water x No. of water molecules in formula X 100%

M r of FeCl3. 6H2O

= 18 X 6 X 100% 270.5

= 39.9%

Step 4: Mass of water in 54.1g of FeCl3. 6H2O

• = 39.9/ 100 X 54.1= 21.6g

Worked Example 5 (cont’d)