1. Formula Mass

2. KI

3. I2

4. Example 1:
H2O

5. Example 2:
(CH3CH2)2O

6. Practice
P4S3
CuCl2
NiF2

7. Percent Composition
“Percent by Mass”

8. Law of Definite Proportions
A compound is always made of the same elements in the same proportion by mass, no matter the size of the sample.
% by Mass: ratio of each element to the total mass of the compound, shown as %.

9. Percent Composition
What percentage of an element is in a chemical compound?
Part x 100
Whole
Use Formula Mass
Percent mass of an element—
Mass of an element in 1 mole of a chemical compound

10. Example 1: Find the percent composition of hydrochloric acid (HCl).

11. Example 2:
Find the percent by mass of magnesium, carbon, and oxygen in magnesium carbonate.
MgCO3
28.83% Mg
14.24% C
56.93%O

12. Example 3:
The formula for DDT is C14H9Cl5. What is the percent by mass of carbon, hydrogen, and chlorine in this compound?
C= %
H= %
Cl= %

13. Example 4:
Osteoporosis is a disease caused by low calcium levels in diets. Calcium tablets can be supplements to diets and are either calcium carbonate, calcium sulfate, or calcium phosphate. Based on the percent by mass of calcium in each chemical compound, which tablet is the most efficient dietary supplement for calcium levels?
CaCO3= % Ca
CaSO4 = %Ca
Ca3(PO4)2 = %Ca