How to Find the Empirical Formula from Percent Composition

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The empirical formula of a chemical compound gives the ratio of elements, using subscripts to indicate the number of each atom. It's also known as the simplest formula. Here is how to find the empirical formula, with an example:

Steps for Finding the Empirical Formula

You can find the empirical formula of a compound using percent composition data. If you know the total molar mass of the compound, the molecular formula usually can be determined as well. The easiest way to find the formula is:

  1. Assume you have 100 g of the substance (makes the math easier because everything is a straight percent).
  2. Consider the amounts you are given as being in units of grams.
  3. Convert the grams to moles for each element.
  4. Find the smallest whole number ratio of moles for each element.

Empirical Formula Problem

Find the empirical formula for a compound consisting of 63% Mn and 37% O

Solution for Finding the Empirical Formula

Assuming 100 g of the compound, there would be 63 g Mn and 37 g O
Look up the number of grams per mole for each element using the Periodic Table. There are 54.94 grams in each mole of manganese and 16.00 grams in a mole of oxygen.
63 g Mn × (1 mol Mn)/(54.94 g Mn) = 1.1 mol Mn
37 g O × (1 mol O)/(16.00 g O) = 2.3 mol O

Find the smallest whole number ratio by dividing the number of moles of each element by the number of moles for the element present in the smallest molar amount. In this case, there is less Mn than O, so divide by the number of moles of Mn:

1.1 mol Mn/1.1 = 1 mol Mn
2.3 mol O/1.1 = 2.1 mol O

The best ratio is Mn:O of 1:2 and the formula is MnO2

The empirical formula is MnO2