the mole 9.5
TRANSCRIPT
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)