Chemistry Ms. Piela A. Percent Composition: the percentage by mass of each element in a compound 1. Looking at the formula for FeO, we might assume the percent compositions of both elements to be 50%. However, because the percent is based on mass, the percent of Fe is 77.7% and oxygen is 22.3% (Oxygen weighs less) B. Examples: 1. Determine the percent composition of the elements in the following compounds: A) H2O B) C6H12O6 C) Ba(NO3)2 C. Practice: A) HF B) NH3 II. Determining Empirical Formulas from Percent Composition A. Empirical Formulas – a chemical formula that shows the composition of a compound in terms of the simplest ratio. B. Steps 1. Convert mass percentage of each element to grams 2. Convert grams to moles using molar mass 3. Compare amounts of moles to find the simplest whole-number ratios Rhyme Time Percent to mass Mass to moles Divide by smallest Multiply until whole C. Examples: Calculate the empirical formula of each of the following substances: 57.14 % C, 6.16% H, 9.52% N, 27.18% O C. Examples 48.0% C, 8.0% H, 28.0% N, 16.0% O D. Practice Calculate the empirical formula of each of the following substances: 1. A compound is 38.77% Cl and 61.23% O. D. Practice 2. Magnetic iron oxide is 72.4% iron and 27.6% oxygen. D. Practice 3. A liquid compound is 18.0% C, 2.26% H, and 79.7% Cl.