1. Composition Stoichiometry
Percent Composition
&
Chemical Formulas

2. • Law of Definite Proportions
– In samples of any chemical compound, the masses of
the elements are always in the same proportions.
– Compares the different elements in a single
compound
– H2O
• mass of H : mass of O is always the same
• Law of Multiple Proportions
– Whenever the same two elements form more than
one compound, the different masses of one element
that combine with the same mass of the other element
are in the ratio of small whole numbers
– Compares the same elements found in different
compounds
– H2O and H2O2
• Mass of O in H2O : mass of O in H2O2 = small whole number

3. Percent Composition
• Percent composition
– Percent by mass of each element in the
compound
– Always the same for a given compound

4. Percent Composition from Mass
Data
• The number of grams of the element
divided by the mass in grams of the
compound
mass of element
% mass of element 100
mass of compound

5. When a 13.60-g sample of a compound containing only
magnesium and oxygen is decomposed, 5.40 g of oxygen
is obtained. What is the percent composition of this
compound?
Mass of oxygen = 5.40 g
Mass of magnesium = 13.60 g – 5.40 g = 8.20 g
Mass of compound = 13.60 g
mass of Mg 8.20g
%Mg 100 100 60.3% Mg
mass of compound 13.60g
mass of O 5.40g
%O 100 100 39.7% O
mass of compound 13.60g

6. Percent Composition from
Chemical Formula
• Number of grams of element in 1 mole of
compound divided by the molar mass of
the compound times 100
mass of elementin 1 molof compound
% mass 100
molarmass of compound

7. Calculate the percent composition of propane (C3H8).
C: 3 x 12.0g = 36.0g / 44.0 X 100 = 81.8%C
H: 8 x 1.0g = 8.0g / 44.0 X 100 = 18.2%H
44.0g
Number of
atoms of
element from
formula Molar mass of
Molar mass of compound
element from
periodic table

8. • Hydrates
– A compound that has a specific number of
water molecules bound to each formula unit
– When writing the formula, use a dot to
connect the formula of the compound and the
number of water molecules per formula unit
• CuSO4 5H2O
– When naming a hydrate, name the compound
then use a prefix followed by the word
“hydrate” to indicate the number of water
molecules per formula unit
• Copper(II) sulfate pentahydrate

9. • Finding the percent of water in a hydrate
mass of water
% H 2O 100
mass of hydrate

10. Calculate the percent by mass of water in washing
soda, sodium carbonate decahydrate.
Formula of compound = Na2CO3 10 H2O
Na : 2 23.0g 46.0g
C : 1 12.0g 12.0g
O : 3 16.0g 48.0g
H 2O :10 18.0g 180.g 286g 100 62.9%H 2O
T otal 286g