2. PERCENT COMPOSITION Section 6.1

3. THE LAW OF DEFINITE PROPORTIONS The elements in a chemical compound are always present in the SAME proportions by mass Ex: H 2 O: the proportion of Hydrogen and Oxygen stays constant

4. MASS PERCENT OF AN ELEMENT % Composition = Mass of element x 100% Mass of compound

5. MASS PERCENT OF AN ELEMENT Mass percent of hydrogen in water: Mass of H 2 = 2 (1.01) = 2.02 g/mol Mass of H 2 O= 18.02 g/mol (Molar mass) % mass of H 2 = mass of H 2 100% mass of H 2 O = 2.02 g/mol x 100% 18.02 g/mol = 11.2 %

6. PERCENT COMPOSITION Tells you the mass percent (%) of all elements in a molecule

7. PERCENT COMPOSITION Ex 1: A compound with a mass of 48.72 g is found to have 32.69 g of zinc and 16.03g of sulfur. What is the percent composition of the compound? Given: Mass of compound 48.72 g Mass of Zn = 32.69 g Mass of S = 16.03 g

8. PERCENT COMPOSITION % Zn= 32.69g x 100% 48.72g % S= 16.03gx 100% 48.72g = 67.10% = 32.9 %

9. PERCENT COMPOSITION Can also be calculated using the chemical formula Ex 2: What is the % composition of each of the elements in C 9 H 8 O? = 9(12.01g/mol) + 8(1.01g/mol) + 16g/mol = 108.09 + 8.08 + 16 = 132.17g/mol

10. PERCENT COMPOSITION % C = 108.09 g x 100% 132.17g = 81.78 % % H = 8.08g x 100% 132.17 g = 6.11 % % O = 16.00g x 100% 132.17 g = 12. 10%

11. PERCENT COMPOSITION Sometimes chemists use % composition when they have unknown compounds to determine the mass of the elements that make up the compound

12. HOMEWORK READ Section 6.1 Page 260: # 1-9 Page 264: # 11-18 Page 266: # 21-30