Percent Composition
This tutorial will help you learn
how to calculate the percentage
by mass of elements in a
compound
What is the percentage of oxygen
in Potassium chlorate?
Place 1.29g of KClO3 in a crucible
and heat it for at least 10 minutes.
Heating this compound will drive the
oxygen atoms out of the compound
and into the air as oxygen gas.
After heating, allow the crucible to
cool to room temperature.
The mass of the contents in the
crucible is now 0.78g. All of the
oxygen has been driven off and only
KCl remains in the crucible. What is
the mass of oxygen that was driven
off?
CORRECT
1.29g – 0.78g = 0.51g of oxygen
Now determine the percentage of
oxygen in the compound by taking
the mass of they oxygen divided by
the mass of the compound and
multiplying by 100. Show your work
on your paper.
GOOD WORK
(0.51g/1.29g)x 100 = 39.1% oxygen
Let’s now do some problems when
we only know the formula of the
compound.
What is the mass of carbon in
carbon dioxide?
#1 – Find the mass of the atom.
Carbon has a mass of 12.0g
#2 – Find the mass of the compound
CO2 has a molecular mass of 44.0g.
#3 – Mass of atom/mass compound
and then multiply by 100.
(12.0/44.0) x 100 = 27.3% carbon
Practice problems
What is the percentage by mass of
iron in Iron(III) chloride?
Iron(III) chloride is FeCl3.
Iron has a mass of 55.8g
FeCl3 has a molecular mass of 162.3
(55.8/162.3) x 100 = 34.4%
Practice problems
What is the percentage by mass of
oxygen in carbon monoxide?
Carbon monoxide is CO.
Oxygen has a mass of 16.0g.
CO has a molecular mass of 28.0g
(16.0/28.0) x 100 = 57.1%
Practice problems
What is the percentage by mass of
chlorine in magnesium chloride?
Magnesium chloride is MgCl2.
The mass of chlorine is 35.5g, so two
chlorines will have a mass of 71.0g.
The mass of MgCl2 is 95.3g.
(71.0/95.3) x 100 = 74.5%