High School Chemistry Notes on Percent Composition

1. Percent Composition
CP Chemistry Unit 7

2. What Percent...
Of the class if female? How do you
know?
Of the class is male? How do you
know?

3. What is percent composition?
• Percent composition is the percent by
mass of each element in a compound
• Formula = mass of element x 100%
mass of compound
• Unit = %
• Round answers to 1 decimal place
(tenth place)

4. How do you solve percent comp?
1.Write compound formula (if not
provided)
2.Do an atom inventory AND calculate
molar mass of compound
3.Use formula
% comp of element = mass of element x 100%
mass of compound

5. Hints
• If there are two elements in a
compound, you will do two separate
percent composition calculations!
• If there are three elements in a
compound, you will do three
separate percent composition
calculations!

6. Practice Problem
Find the percent composition of each
element in dihydrogen monoxide.
Chemical Formula:
Atom Inventory and Molar Mass:
Formula:

7. Practice Problem
Percent hydrogen in water
11.1% H in H2O
Percent oxygen in water
88.9% O in H2O

8. This means...
In water there is,
11.1% Hydrogen
88.9% Oxygen
The totals should VERY CLOSELY total
100% (sometimes not exactly, since
you rounded answers)