Example: What are the empirical and molecular formulas of a compound that contains 40.9%
carbon, 4.58% hydrogen, 54.52% oxygen, and has a molecular weight of 264 g/mol?
Solution: First, assume that we have a sample that weighs 100 g total. The percentage then
translates directly into the weight of that element in the sample (e.g., 40.9% by
weight means 40.9 g in a 100-g sample). Then convert grams to moles by dividing
the weight of each element by its molar atomic mass:Next, find the simplest whole number ratio of the elements by dividing the number
of moles by the smallest number obtained in the previous step.Finally, the empirical formula is obtained by converting the numbers obtained into
whole numbers (multiplying them by an integer value). In this case, we want to turn
1.33 into an integer; the smallest number we can multiply it by to make it an integer
is 3:1.33 × 3 = 4
The empirical formula is therefore 3 × C 1 H1.33O 1 = C 3 H 4 O 3.This method gives the empirical formula because the elements are always in their smallest whole
number ratio. A molecular formula of C 6 H 8 O 6 , which is a multiple of two of the empirical formula,