Percent composition is the percent by mass of each element in a compound, calculated by taking the mass of an element divided by the total mass of the compound and multiplying by 100. To calculate percent composition, you write the compound formula, do an atom inventory and find the molar mass, then use the formula: % composition of element = mass of element x 100% / mass of compound. For compounds with multiple elements, separate calculations must be done for each element. As an example, the percent composition of hydrogen in water is 11.1% and oxygen is 88.9%, which should total very closely to 100%.

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)