1. Section 9.3—Analysis of a Chemical Formula
How can we determine a chemical formula?

2. Percent Composition
How is percent composition found?

3. What is Percent Composition?

4. Example #1
Example:
An 8.5 g sample is composed of carbon and hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of the carbon and hydrogen in the sample?

5. Example #1 If total = 8.5 g and C = 5.5 g - then H = 3.0 g Example:
A 8.5 g sample is composed of carbon and hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of the sample?
If total = g
and C = g -
then H =
3.0 g

6. Let’s Practice #1
A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample?

7. Let’s Practice #1
A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample?
27.5 g 
 27.5 g
100
100

8. Example #2
Percent composition can also be determined from a chemical formula
Example:
Find the percent composition, by mass, of CaCl2

9. Example #2
Percent composition can also be determined from a chemical formula
Example:
Find the percent composition, by mass, of CaCl2 Ca Cl 1 2
40.08 g/mole
35.45 g/mole  =
70.90 g/mole +
g/mole
So for 1 mole:
total = g
Ca = g
Cl =
70.90 g

10. Let’s Practice #2
Find the percent composition, by mass, of NaNO3

11. Find the percent composition, by mass, of NaNO3
Let’s Practice #2
Find the percent composition, by mass, of NaNO3 Na N 1
22.99 g/mole
14.01 g/mole  = +
85.00 g/mole O 3
16.00 g/mole
48.00 g/mole