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# Video:Empirical Formula

with Anne Marie Helmenstine

The empirical formula of a compound is its simplest chemical formula, expressed as a ratio of elements in the compound. Conquer your chemistry fears and learn how to use the empirical formula easily.See Transcript

## Transcript:Empirical Formula

The empirical formula of a compound is its simplest chemical formula, expressed as a ratio of elements in the compound. 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.

### To Use the Empirical Formula Easily:

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

### Empirical Formula Example:

As an example, here’s how we can find the empirical formula for a compound consisting of 63% Mn and 37% O. 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.

To calculate number of grams per mole, for 63 grams of Mn, we will multiply 63 by the result of 1 mole Manganese divided by 54.94 grams manganese. You get 1.1. Similarly, for 37 grams of Oxygen we will multiply 37 by the result of 1 divided by 16, for 16 grams of oxygen. You get 2.3.

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. So, 1.1 divided by 1.1 gives 1. And 2.3 divided by 1.1 gives 2.1. The best ratio is Mn:O of 1:2 and the formula is MnO2. The empirical formula is MnO2.