1. Percentage Composition
Chemistry 11
Ms. McGrath

2. Percentage Composition
Chemists use molar mass to find out important information about compounds. Chemists can analyze the percent composition of compounds to figure out how to make certain things cheaper in a laboratory.

3. Percentage Composition The Law of Definite Proportions
The elements in a given compound are always present in the same proportions.

4. Percentage Composition
The molecular mass, of a molecule, or the formula mass, of an ionic compound, are determined by finding the sum of the atomic mass of all the atoms that make up the compounds. e.g. molecular mass of H2O is u 2(1.01 u) + 1(16.01 u) = u

5. Percentage Composition
How much of this mass was contributed by the
hydrogen atoms? By the oxygen atom?
This is determined by finding the
PERCENT COMPOSITION.

6. Percentage Composition
% H = mass of H atoms in one molecule total mass of H2O molecule = 2.0 u x 100% = 11.1% 18.0 u % O = mass of O atoms in one molecule = 16.0 u x 100% = 88.9%

7. Percentage Composition
Since the sum of the two percentages (11.1% and 88.9%) add up to 100%, there is confidence that no mathematical error was made. Water, therefore, is composed of: 11.1% Hydrogen and 88.9% Oxygen.

8. Percentage Composition
What is the percent composition of calcium hydrogen carbonate, Ca(HCO3)2? Total Formula Mass: u %Ca = (40.1 u ÷ u) x 100% = 24.7% %H = (2.0 u ÷ u) x 100% = 1.2% %C = (24.0 u ÷ u) x 100% = 14.8% %O = (96.0 u ÷ u) x 100% = 59.2% Total: 99.9% (which is close enough to 100%)

9. Percentage Composition
Complete questions #1 – 4 page 82
Complete questions #5 – 8 page 85
Complete questions #1, 3, 4, 5, 6 page 86
Complete Thought Lab, page 83, to be handed today