# How can an empirical formula be interpreted?

May 1, 2016

For example, a question has been given to you.
Example question: Calculate the empirical formula of a compound that contains 11% hydrogen and 89% oxygen by mass.

First step:
Assume that percentages are masses.
Assume that 100 grams contains 11 grams of hydrogen and 89 grams of oxygen.

Second step:
Use the formula $n = m \div M$ to calculate the number of moles (n) of each atom.

The number of moles (n) in hydrogen is:
n = 11 grams $\div$1 g/mol = 11 moles.
The number of moles (n) in oxygen is:
n = 89 grams $\div$16 g/mol = 5.6 moles.

Third step:
Divide all number of moles (n) by the SMALLEST, then round to nearest whole number.

Hydrogen (H): 11 moles $\div$ 5.6 moles = 2.00
Oxygen (O): 5.6 moles $\div$ 5.6 moles = 1.00

Last step:
Round to whole numbers, and assign as subscripts.
The empirical formula of this compound is ${H}_{2} O$.