Percentage Composition – Text pgs. 198-204
The Law of Definite Proportions explains important observations made about compounds:

The elements in a _____________________ are always present in the same proportions by mass.
(proposed by Joseph Louis Proust)
For example, the masses of hydrogen and oxygen in pure ___________ always exist in a ratio of 2g :16 g
The mass of an element in a compound, expressed as a % of the total mass of the compound, is the
element’s mass percent.
Example 1: The mass percent of hydrogen in water (from any source) is
and the mass percent of oxygen in water is always
11.2%
88.8%
However, the same elements can make up different compounds. Ie. H20 & H2O2, each compound having a
different mass percent of oxygen and hydrogen.
Example 2: The mass percent of hydrogen in H2O2 is
5.9%
and the mass percent of oxygen in H2O2 is always
94.1%
% composition can be determined experimentally, and then used to help _________________ the compound.
Percent composition is the percent mass of each element present in a compound. It is calculated in two
ways:
1) Experimental % Composition (Unknown chemical formula, use masses measured in the lab):
% mass = Element mass
x 100%
Compound mass
2) Theoretical % Composition (Known chemical formula, uses the atomic masses from the __________):
% mass = Total Element atomic mass in compound formula x 100%
Compound atomic mass (all atoms in formula)
useful when you want to extract a certain element from a compound.
For example,
Mercury is most often found in nature as mercury(II) sulfide, HgS. Knowing the ____________ composition of HgS
helps a metallurgist predict the mass of mercury that can be extracted from a sample of HgS.
Sample Problem – Experimental % Composition from Mass Data (p. 200)
A sample of a compound has a mass of 48.72 g. The sample is found to contain 32.69 g of zinc and 16.03 g of
sulfur. What is the percentage composition of the compound?
Practice Problems, Pg. 201 #2-4
Sample Problem – Finding Percentage Composition from a Chemical Formula (p.203)
Cinnamaldehyde, C9H8O, is responsible for the characteristic odour of cinnamon. Determine the percentage
composition of cinnamaldehyde by calculating the mass percents of carbon, hydrogen, and oxygen.
Practice Problems, Pg. 204 #5-8
Challenge Practice: A compound with a mass of 40 g has 21.54 g of iron & 18.46 g of sulfur. What is the
name of this compound ?
Challenge Practice 2: How much carbon is present in 15.2 g of carbon dioxide gas?