Empirical Formula Tutorial – How To Calculate The Empirical Formula

The empirical formula, also known as the simplest formula of a chemical compound is a representation of the simplest ratio of elements that make up the molecule. These ratios are denoted by subscripts next to the element symbols.

For example:  The molecular formula for hydrogen peroxide is H2O2. This means there are two hydrogen atoms and two oxygen atoms in one molecule of hydrogen peroxide. The ratio between these elements is 1:1, For every hydrogen atom present in the molecule, there is one oxygen atom. The empirical formula is HO.

This example problem will guide you through the steps to determine the empirical formula of a compound.

Example Problem:
A sample of a compound is found to contain 63.5% silver, 8.2% nitrogen and 28.3% oxygen. What is the empirical formula for this compound?

Solution:
Step 1: Assume you have 100 grams of the sample. This makes it easier to determine the amounts in grams for each element in the compound from the given % compositions.

100 grams of the compound will have:
63.5 grams of silver
8.2 grams of nitrogen
28.3 grams of oxygen

Step 2: Convert these weights into moles.

Use a periodic table to get the atomic weights of each element.
Silver: 107.87 g/mol
Nitrogen: 14.01 g/mol
Oxygen: 16.00 g/mol

Divide the sample weight by the atomic weight to get the number of moles of each element
Silver:
mole calculation of silver
Nitrogen:
mole calculation of nitrogen
Oxygen:
mole calculation of oxygen

Step 3: Determine the smallest whole number ratio between these values.

The best place to start is to find the smallest number of moles. In this case, it is silver and nitrogen at 0.59 moles. Divide each element’s amount by this number.

Silver:
empirical formula ratio math 1
Nitrogen:
empirical formula ratio math 1
Oxygen:
empirical formula ratio math 2

For every mole of silver there is one mole of nitrogen and 3 moles of oxygen.
The empirical formula is then AgNO3.

Answer:
The empirical formula for this compound is AgNO3.

Empirical Formula Tutorial – How To Calculate The Empirical Formula
Last modified: September 16th, 2014 by Todd Helmenstine