Finding formulas of compounds

You can use the mole concept to calculate the empirical formula of a compound using the percentage composition data for that compound — the percentage by weight of each element in the compound. (The empirical formula indicates the different types of elements in a molecule and the lowest whole-number ratio of each kind of atom in the molecule).

When I try to determine the empirical formula of a compound, I often have percentage data available. The determination of the percentage composition is one of the first analyses that a chemist does in learning about a new compound. Here’s how to find an empirical formula using moles and the percentages of each element:

1. Assume you have 100 grams of the compound so you can use the percentages as weights; then convert the weight of each element to moles.

For example, suppose you determine that a particular compound has the following weight percentage of elements present: 26.4 percent Na, 36.8 percent S, and 36.8 percent O. Because you’re dealing with percentage data (amount per hundred), assume that you have 100 grams of the compound so you can write the percentages as weights. Then convert each mass to moles, like this:

2. Write the empirical formula, changing subscripts to whole numbers if necessary.

Now you can write an empirical formula of Na_1.15S₁.₁₅O_2.30. Your subscripts have to be whole numbers, so divide each of these by the smallest, 1.15, to get NaSO₂. (If a subscript is 1, it’s not shown.) You can then calculate a weight for the empirical formula by adding together the atomic masses on the periodic table of one sodium (Na), one sulfur (S) and two oxygen (O). This gives you an empirical formula weight of 87.056 grams.