Empirical & Molecular Formulas

Percent Composition Def – the percent by mass of each element in a compound Percent by mass = mass of element x 100 mass of compound Example: Find the percent composition of the following compound: NH 4 Cl N = 26.1%, H = 7.5%, Cl = 66.4% Make sure all percentages equal 100%!!

Empirical Formula Def – the smallest whole number mole ratio of each element in a compound Assume that all percentages are out of 100 g, meaning the percentages can now be the amount of grams (assuming there is 100 grams present) Example: The percent composition of a compound is 40.05% S and 59.95% O. What is the empirical formula of the compound? SO 3

Molecular Formula Part I Def – the actual number of atoms of each element in one molecule or formula unit of the substance Molar mass of the unknown formula MUST be given!! n = experimentally determined molar mass mass of empirical formula Multiply the empirical formula by n to get the molecular formula [molecular formula = (empirical formula) x n] Example: A colorless liquid composed of 46.6% N and 53.32% O has a molar mass of g/mol. What is the molecular formula?

Molecular Formula Part II If you are not given percent composition, you must find them first and then proceed Example: A compound was found to contain g of carbon and g hydrogen. The molar mass of the compound is g/mol. Determine the molecular formula.