3. Percentage Composition
Chapter 8
Percentage Composition

4. Water is made of hydrogen and oxygen
Water is made of hydrogen and oxygen. How many grams of each are in the sample of water below?

5. Percent Composition % =
Percent composition is the percent by mass of an element within a compound.
Divide the mass of each element by the formula mass of the compound
Mass of element
% =
X 100
Formula Mass of compound

6. Percent Composition of H2O
2 X H = 2 X 1.0 g = 2.0 g
1 X O = 1 X 16.0 g = 16.0 g
gram formula mass = 2.0 g g = 18.0 g
% H = 2.0g X 100 = 11.2%
18.0g
% O = 16.0g X 100 = 88.8%

7. Percent Composition of H2O
2 X H = 2 X 1.0 g = 2.0 g
1 X O = 1 X 16.0 g = 16.0 g
gram formula mass = 2.0 g g = 18.0 g
% H = 2.0g X 100 = 11.2%
18.0g
% O = 16.0g X 100 = 88.8%
What does this mean?

8. Pure water’s mass is 11.2% hydrogen and 88.8% oxygen.
(0.112)(128.98g) = 14.45g H
(.888)(128.98g) = g O or
128.98g – 14.45g = g O
11.2% H
88.8% O

9. Percentage Composition
Remember that you have to calculate the formula mass!
The problem doesn't say to do so...

10. Determine the Percent Composition of C6H12O6

11. Determine the Percent Composition of C6H12O6
6 X C = 6 X 12.0 g = 72.0g
12 X H = 12 X 1.0g = 12.0g
6 X O = 6 X 16.0g = 96.0g
gfm = grams/mole

12. Determine the Percent Composition of C6H12O6
6 X C = 6 X 12.0 g = 72.0g
12 X H = 12 X 1.0g = 12.0g
6 X O = 6 X 16.0g = 96.0g
gfm = grams/mole
% C =
% H =
% O =
(72.0g/180.0g) X 100 = 40.0%
(12.0g/180.0g) X 100 = 6.67%
(96.0g/180.0g) X 100 = 53.3%

13. How many grams of carbon are in 156g C6H12O6
6 (156g)(0.400) = 62.4g carbon 12 6
Molar
% C =
% H =
% O =
(72.0g/180.0g) X 100 = 40.0%
(12.0g/180.0g) X 100 = 6.67%
(96.0g/180.0g) X 100 = 53.3%

14. What is this? a

15. Iron Pyrite “Fool’s Gold” (FeS2)a

16. How many grams of sulfur are in this 28.8g sample of iron pyrite FeS2?

17. How many grams of sulfur are in this 28.8g sample of iron pyrite FeS2?

18. How many grams of sulfur are in this 28.8g sample of iron pyrite FeS2?
( 32.1) = g/mol
%S = 64.2/120.0 x 100= 53.5%S
28.8g x = 15.4g S a

19. Homework (Due in 2 days).
Percentage Composition Worksheet