# Percent Composition Empirical Formulas Molecular Formulas Percent Composition

• Slides: 7

Percent Composition, Empirical Formulas, Molecular Formulas

Percent Composition • Percent Composition – the percentage by mass of each element in a compound (*note: not the ratio of elements in the compound) • Law of Definite Proportions – compounds always contain the same proportions of elements by mass. Part _______ Percent = x 100% Whole So… Percent composition Mass of element in 1 mol of a compound or = __________ x 100% Mass of 1 mol of compound molecule

Formulas Percent composition allow you to calculate the simplest ratio among the atoms found in compound. Empirical Formula – formula of a compound that expresses lowest whole number ratio of atoms. Molecular Formula – actual formula of a compound showing the number of atoms present Examples: C 4 H 10 - molecular C 2 H 5 - empirical C 6 H 12 O 6 - molecular CH 2 O - empirical

Formulas Is H 2 O 2 an empirical or molecular formula? Molecular, it can be reduced to HO HO = empirical formula

Calculating Molecular Formula A white powder is analyzed and found to have an empirical formula of P 2 O 5. The compound has a molar mass of 283. 88 g/mol. What is the compound’s molecular formula? Step 3: Multiply Step 1: Molar Mass P = 2 x 30. 97 g = 61. 94 g O = 5 x 16. 00 g = 80. 00 g 141. 94 g Step 2: Divide MM by Empirical Formula Mass 238. 88 g =2 141. 94 g (P 2 O 5)2 = P 4 O 10

Calculating Molecular Formula A compound has an experimental molar mass of 78 g/mol. Its % composition is found to be 92. 24% carbon and 7. 76% hydrogen. What is its molecular formula of this compound? First, determine the empirical formula using the % composition. Empirical formula of this compound is CH. Determine the multiple C = 12. 01 g/mol H = 1. 01 g/mol 13. 02 g/mol 78 g/mol 13. 02 g/mol =6 (CH)6 = C 6 H 6

Homework: Page 298 #1 Page 300 #2