Empirical and molecular formulae

Last time

Before we start: last time, in Atoms, ions and isotopes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Same ratio, different molecules

Ethene is C₂H₄, and propene is C₃H₆. Both have exactly twice as many hydrogen atoms as carbon atoms. The simplest ratio of atoms, here CH₂, is called the empirical formula. And you can find it from experimental masses.

Two kinds of formula

The molecular formula shows the actual number of atoms of each element in one molecule. Glucose is C₆H₁₂O₆. The empirical formula shows the simplest whole-number ratio of the atoms. For glucose, that is CH₂O. For many compounds, they are different. For ionic compounds, such as NaCl, the formula is already empirical.

Molecular to empirical

To find the empirical formula from a molecular formula, divide every number by the biggest number that goes into all of them. C₆H₁₂O₆: six, twelve and six all divide by six. So CH₂O. C₂H₄O₂, ethanoic acid: divide by two, giving CH₂O. H₂O₂: divide by two, giving HO. And C₈H₁₈ divides by two, giving C₄H₉.

Empirical from masses

First, write down the mass, or the percentage, of each element. Second, divide each by its Ar. This gives the relative number of moles. Third, divide all the answers by the smallest one. Fourth, if you get a half, multiply everything by two. If you get a third, multiply by three. Fifth, write the formula with these whole numbers.