Key concepts in chemistry Study Guide

The empirical formula represents the simplest whole-number ratio of atoms in a compound. It can be determined from experimental data such as reacting masses or percentage composition. To find the empirical formula, you first convert the masses of each element into moles using their atomic masses. Then, you divide the number of moles of each element by the smallest number of moles calculated to find the simplest mole ratio. If the ratios are not whole numbers, you multiply all ratios by a common factor to convert them into the smallest possible whole numbers. This final ratio gives the subscripts for each element in the empirical formula. This method is fundamental in understanding the basic composition of compounds.

The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound. It is deduced from experimental data, such as percentage composition by mass or by converting the molecular formula into its simplest ratio. For example, if a molecule contains 6 carbon atoms and 6 hydrogen atoms, its molecular formula is C₆H₆, but the simplest whole-number ratio between carbon and hydrogen atoms is 1:1, making the empirical formula CH. The molecular formula, on the other hand, shows the actual number of atoms of each element in a molecule. To determine the molecular formula from the empirical formula, one needs the relative molecular mass (M₉) of the compound. The molecular formula is always a whole-number multiple of the empirical formula. This multiple is found by dividing the relative molecular mass of the compound by the relative formula mass of the empirical formula.