1. Percent Composition

2. Percent composition of a compound refers to the percentage by mass.
In other words, the percentage composition of water shows what % (by mass) is Hydrogen, and what % is Oxygen.
Percentage composition by mass does not tell us the ratio by which the atoms combine. (Does not tell us the subscripts)

3. % Example
Jim goes to a school that has 258 total students.  If 164 of the students are boys, what percentage of the student body is made up of girls?

4. In this example, you found the percent composition by dividing one part by the total and multiplying by 100.
               part
% composition = x 100        whole    

5. Finding the percentage composition by mass works the same way.
Divide the mass that one element contributes to the compound by the mass of the entire compound. 
                                                                                                  mass from element
% Composition = x 100                          total mass of the compound

6. Example
What percentage of the mass of carbon dioxide (CO2) is made up by the carbon?
Mass of Carbon = x 1 = amu
Mass of Oxygen = x 2 = amu

7. Continued part % comp. of Carbon = --------------- x 100 whole
12.01 amu
44.01 amu
Answer = 27.3%

8. Example 4 What is the percentage composition of glucose (C6H12O6) ?
Start by determining the molar mass!
C = x 6 = amu
H = 1.01 x 12 = amu
O = x 6 = amu
amu

9. Remember!!! part % comp. of Element = --------------- x 100 whole
72.06 amu % for Carbon = x 100
                         amu
40.00% C

10. 12.12 u % for Hydrogen = ---------------- x 100
                         u % H
96.00 u % for Oxygen = x 100
                         u % O

11. Check your Answer!
One way to check your answer is to make sure that all of the percentages add up to approximately 100%.  (i.e. 40.0% % % = 100%)  
Your total may be off by a few tenths of a percent, due to rounding.