Review Molar relationship and
Molar conversion problems
Percent Composition and Chemical
Formulas
Agenda:
1. Review molar relationships
2. Review molar conversion problems
3. Intro to Percent Composition
4. Guided Practice
5. Independent Practice
Objective:
My words: SWBAT use percent composition of compounds to
formulate empirical and molecular formulas of compounds
as demonstrated through a handout/worksheet.
Your Words:…
Why learn this…
 Many times scientists are faced with unknown compounds in
the lab. There is a white powder…what is it? Can you think
of a time when an unknown compound was the cause of mass
panic?
 Experimentally determining the chemical formula so we
know what the compound is can actually save lives!
Connecting to you…
 Is your shirt made of 100% cotton or wool? Why or
why not?
 Usually a shirt’s composition is a bunch of different
types of materials…
 Also, if you have experience with lawn care, you
may know that percents of different elements in the
mixtures of fertilizers is important in different
seasons. For example, you may use nitrogen rich
fertilizer in the spring to green the grass and a
fertilizer with a higher percentage of potassium in
the fall to strengthen the root systems.
Percent Composition
 The relative amounts of elements in a compound can be
expressed as the percent composition (or percent by mass) of
each element in the compound.
Percent Composition
 The percent by mass of an element in a compound is the
number of grams of the element divided by the mass (g) of
the compound. Then multiplied by 100%.
Sample Problem 1
When a 13.6 g sample of a compound containing only
magnesium and oxygen is decomposed, 5.4 g of oxygen is
obtained. What is the percent composition of this
compound?
Sample Problem 2
A compound is formed when 9.03 g of Mg combines with 3.48
g of N. What is the percent composition of this compound?
Sample Problem 3
When a 14.2 g sample of Mercury (II) Oxide is decomposed
into its elements by heating, 13.2 g of mercury are obtained.
What is the percent composition of the compound?
Percent composition from the chemical
formula
 Let’s say you are not trying to find the formula but rather,
have the formula in front of you!
 You can find the percent composition by mass of the
compound by using the formula and the molar mass!
Sample Problem 4
Propane (C3H8), the fuel commonly used in gas grills, is one of
the compounds obtained from petroleum. Calculate the
percent composition of propane.
Sample Problem 5
Calculate the Percent composition of baking soda: Sodium
hydrogen carbonate (sodium bicarbonate).
Sample Problem 6
Calculate the percent of nitrogen in the following common
fertilizers:
a. NH3
b. NH4NO3
Recap
 How do you calculate the percent by
mass of an element in a compound when
you know the exact masses but do not
know the formula?
 How do you calculate the percent by
mass of an element in a compound when
you know the formula?
 What is the percent composition of
CO2?