PERCENT COMPOSITION Section 6.1

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

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

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= g/mol (Molar mass) % mass of H 2 = mass of H 2 100% mass of H 2 O = 2.02 g/mol x 100% g/mol = 11.2 %

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

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

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

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 = = g/mol

PERCENT COMPOSITION % C = g x 100% g = % % H = 8.08g x 100% g = 6.11 % % O = 16.00g x 100% g = %

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

HOMEWORK READ Section 6.1 Page 260: # 1-9 Page 264: # Page 266: # 21-30