Ch. 7-3 Percent Composition

POINT > Define percent composition POINT > Calculate percent composition POINT > Use molar mass as a conversion unit (review)

POINT > Define percent composition Percent composition is the percentage, by mass, of each element in a compound The Law of Definite Proportions tells us that the percent composition of a chemical compound does not vary from sample to sample

POINT > Calculate percent composition Percent composition can be calculated from the chemical formula of a compound using molar masses: mass of element in 1 mol of compound % comp of element = 100% x molar mass of compound

POINT > Calculate percent composition Ex. Calculate the percent composition of water Molar mass of H2O = 18.0 g/mol Molar mass of O in 1 mol H2O = 16.0g/mol Molar mass of H in 1 mol H2O = 2.0g/mol % mass of O 16.0g/mol = x 100% = 88.9% O 18.0g/mol 11.1% H

WB CHECK: Calculate the percent composition of methane: Molar mass of CH4 = 16.0 g/mol Molar mass of C in 1 mol CH4 = 12.0g/mol Molar mass of H in 1 mol CH4 = 4.0g/mol 25.0% H 75.0% C

POINT > Calculate percent composition If the formula is unknown, percent composition can be calculated from experimental data: % comp of element mass of element (g) = 100% x mass of compound (g)

POINT > Calculate percent composition Ex. An unknown compound contains only sodium and chlorine. Analysis shows that a 25.0g sample has 15.2 g Cl. What is the percent composition of the compound? 15.2g Cl 25.0 g compound 39.2% Na x 100% = 60.8% Cl

WB CHECK: A compound is formed when 1.90g of hydrogen combine with 23.1g of carbon and 27.0g of nitrogen. What is the percent composition of this compound? 3.65% H 44.4% C 51.9% N

POINT > Calculate percent composition If you know the percent composition, you can use it to determine the mass of an element in a compound percent comp of element Z mass of element Z mass of compound = x

POINT > Calculate percent composition Ex. Calculate the mass of oxygen in 45.0g of water. Water is 88.9% O 45.0g H2O x .889 O = 40.0g Oxygen

WB CHECK: Calculate the mass of carbon in 87.3g of methane. 65.5g Carbon

POINT > Use molar mass as a conversion unit (review) How many molecules in 77.5g of methanol (CH3OH) 77.5g x = 2.42 mol CH3OH 2.42 mol CH3OH x = 1.46 x 1024 molecules CH3OH 1 mol 32.0g 6.02 x 1023 molecules 1 mol

Homework Read 7.3 pages 225-232 Practice #1-2 page 227 Practice #1-3 page 230 Practice #1-3 page 232 F.A. #1-6 page 232 % Comp W.S.