Calculating Empirical Formulas

Empirical formulas are essential in general chemistry courses, and are usually taught early in a chemistry sequence. The principles of empirical formulas are relevant for most further chemistry studies, so it is an important topic to master efficiently.

A compound or molecule is described with a formula that states exactly how many atoms there are in one mole of each element. An empirical formula is the simplest possible formula for this compound that keeps the ratios between the elements constant.

As an example consider H2C6. The ratio between hydrogen and carbon is 1 to 3. If we did not need to look at exactly how many hydrogen and carbon atoms are present in one mole of the compound, we could say that the easiest way to write this formula is HC3. From this formula we see that the ratio is still 1 to 3, and this is called an empirical formula.

Now suppose we had a compound, and we were given the percent composition of the elements. The question asked for the empirical formula. The strategy we should apply involves looking at our percent data as though we had a total mass of 100g. If there is 30 % of an element present, we can assume that if the total mass is 100 grams, we have 30 grams of that element. By looking up the molar mass of the element you can find the number of moles.

If we do this to every element that we know the percent composition for, we can then divide everything by the lowest number of moles, so that at least one of the elements will be of one mole. The ratios of each element will be the empirical formula.

Most of these problems have fairly easy self-checks. When you divide by the lowest number of moles you should get numbers which could be multiplied and converted to whole numbers. Even in your empirical formula you do not want fractions, so if you cannot get integer ratios of moles you should check over your work to make sure that everything is computed correctly.

Empirical type problems are almost always present on a general chemistry 1 final exam, and it is not of the most difficult problems to tackle. The main idea to remember is that if you have 100% total of something, you could assume that you had 100 grams, or 1000 grams present, and then calculate the masses of the rest by using the percentages.