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 H 2 O is 18.0 u 2(1.01 u) + 1(16.01 u) = 18.02 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 H 2 O molecule = 2.0 u x 100% = 11.1% 18.0 u % O = mass of O atoms in one molecule total mass of H 2 O molecule = 16.0 u x 100% = 88.9% 18.0 u

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(HCO 3 ) 2 ? Total Formula Mass: 162.1 u %Ca = (40.1 u ÷ 162.1 u) x 100% = 24.7% %H = (2.0 u ÷ 162.1 u) x 100% = 1.2% %C = (24.0 u ÷ 162.1 u) x 100% = 14.8% %O = (96.0 u ÷ 162.1 u) x 100% = 59.2% Total: 99.9% (which is close enough to 100%)

9. Percentage Composition Review the Sample Problem on pages 81 and 82. Complete questions page 82, #1 – 4.