The empirical formula or simplest formula of a chemical compound is the simplest ratio of elements that make up the molecule. These ratios are denoted by subscripts next to the element symbols. This example problem will guide you through the steps to determine the empirical formula of a compound.
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 of hydrogen peroxide is HO.
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:
Nitrogen:
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:
Nitrogen:
Oxygen:
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.