1. Percent Composition From a Chemical Formula Chemistry 11 Ms. McGrath

2. Percentage Composition From a Chemical Formula Percentage composition Refers to the relative mass of each element in a compound.

3. Percentage Composition From a Chemical Formula We previously learned how to determine percentage composition using mass. This is helpful when we do not have the chemical formula. Often we use the chemical formula to determine the percentage composition. This is useful when extracting a certain element from a compound.

4. Percentage Composition From a Chemical Formula We use the molar mass and the chemical formula of the compound to calculate its percentage composition. Example: We want to determine the percentage composition of HgS. We can assume that we have one mole of HgS to find the mass percents of mercury and sulfur.

5. Percentage Composition From a Chemical Formula Mass percent of HgS = Mass of Hg in 1 mol HgS x 100% Mass of 1 mol of HgS = 200.59 g x 100% 232.66 g = 86.22% Mercury (II) sulfide is 86.22% mercury by mass. Since there are only two elements in HgS, we can subtract the mass percent of mercury from 100% to find the mass percent of sulfur. Mass percent of S in HgS = 100% - 86.22% = 13.78%

6. Percentage Composition From a Chemical Formula You try! What is the percentage composition of hydrogen peroxide, H 2 O 2 ?

7. Percentage Composition From a Chemical Formula You try! What is the percentage composition of hydrogen peroxide, H 2 O 2 ? % H = Mass of H in 1 mol H 2 O 2 x 100% Mass of 1 mol of H 2 O 2 = 2.02 g x 100% 34.02 g = 5.94% Now, lets calculate oxygen…

8. Percentage Composition From a Chemical Formula Now, lets calculate oxygen… % O = Mass of O in 1 mol H 2 O 2 x 100% Mass of 1 mol of H 2 O 2 = 32.00 g x 100% 34.02 g = 94.06%

9. Percentage Composition From a Chemical Formula See sample problem page 84, 85 Try page 85 #5 - 8