Percent Composition and Empirical Formula

Percent Composition and Empirical Formula 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.

Percent Composition and Empirical Formula