Percentage Composition
Law of Definite Proportions – the elements in a
chemical compound are always present in the same
proportions by mass.
For example, H2O, water is made up of 2 atoms of
hydrogen bonded to one atom of oxygen, no matter
where that water comes from…a lake, a drinking fountain,
melted snow at the top of a mountain or even a toilet…it’s
H2O of course!
We know that the same elements can combine in
different proportions, but that will make a different
compound, with very different properties from each other.
For example, Hydrogen can combine with Oxygen as:
H2O = water
H2O2 = hydrogen peroxide
Although both compounds may look similar (clear,
colourless, liquids), they have very different chemical
properties…they also have different molar masses:
MH2O
= 2(1.01 g/mole) + 1(16.00 g/mole)
= 2.02 g/mole + 16.00 g/mole
= 18.02 g/mole
MH2O2 = 2(1.01 g/mole) + 2(16.00 g/mole)
= 2.02 g/mole + 32.00 g/mole
= 34.02 g/mole
Percentage Composition – of a compound is the
relative mass of each element in the compound
(expressed as a percent).
To calculate the percentage composition for a compound,
you must first find the Molar Mass (M) of the compound,
then find the mass percent of each element.
Example: Find the percentage composition of H2O.
We already calculated the Molar Mass of H2O
MH2O = 18.02 g/mole
Now we find the mass percent of each element in H2O.
So, there are 2 atoms of H in one molecule of water (or 2
moles of atoms of H in 1 mole of H2O) and the Molar
Mass of H is 1.01 g/mole, so 2 moles of H weighs
2 x 1.01 g/mole. = 2.02 g/mole
Mass percent of H in H2O = 2.02 g/mole x 100%
18.02 g/mole
= 11.2%
Now calculate the mass percent of Oxygen in H2O:
Mass percent of O in H2O = 16.00 g/mole x 100%
18.02 g/mole
= 88.8%
So, water is 11.2% Hydrogen by mass and 88.8%
Oxygen by mass.
Example 2: Find the percentage composition of H2O2.
We already found the Molar mass of hydrogen peroxide.
MH2O2 = 34.02 g/mole
Mass percent of H in H2O2 = 2(1.01 g/mole) x 100%
34.02 g/mole
= 2.02 g/mole x 100%
34.02 g/mole
= 5.9%
Mass percent of O in H2O2 = 2(16.00 g/mole) x 100%
34.02 g/mole
= 32.00 g/mole x 100%
34.02 g/mole
= 94.1%
Therefore the percentage composition f hydrogen
peroxide is 5.9% hydrogen and 94.1% oxygen.
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Homework: Page 204 Q 6 and Q 7