Understanding the Empirical Formula in Chemistry

The empirical formula was discovered by scientists in the mid-1800's, and for the first time, it allowed chemists to express what was going on with materials common and rare on a microscopic level.

These chemists would derive empirical formulas for specific chemical compounds in the lab by separating them into the individual elements that we can find on the periodic table and measuring the relative amounts present.

Example of a Chemical Compound

For instance, table salt (sodium chloride) can be divided into the pure elements present, which are sodium (NA) and chlorine (Cl) atoms. Individually these elements are highly unstable and tend to react with oxygen and water present in this atmosphere.

They're also poisonous if consumed on their own, but mixed together, they create a molecular compound which an essential part of our daily diet.

To arrive at the empirical formula for a compound like table salt, chemists would have to start a reaction separate it into its base atoms.

Assuming that 100 percent of the material had reacted, they could then weigh the molar mass of the sodium left over versus the molar mass of the chlorine to find the empirical makeup of the compound.

In the case of table salt, the mass of sodium to chloride is a mole ratio of 1:1, giving an empirical formula of NaCl, which also happens to be the correct chemical formula for the same compound.

Back in the 19th century, the methods used to split compounds into their atomic ingredients and find the resulting empirical formula were revolutionary in helping people understand what goes into creating chemicals and why they exhibit certain characteristics.

However, the empirical formula doesn't tell the full story about what's truly going on inside a molecule. More advanced chemical formulas had to be developed.

In labs today — as well as in the study of chemistry — you are much more likely to bump into the molecular formula for a compound rather than the empirical formula. The reason for this is that we now know that individual molecules tend to be more complex than the smallest whole number ratio of atoms within them.

Going by empirical formula alone, we would also end up with different compounds sharing the same formula, which isn't very useful when trying to categorize chemical properties. The total mass of a molecule could be much larger than its empirical formula would suggest.

Table salt is a very convenient compound to measure when it comes to empirical formula because it contains exactly one sodium and one chlorine atom per molecule. Not all compounds divide so perfectly, and knowing the simplest ratio of mass inside the molecule may not tell us exactly what is actually a single molecule.

What Molecular Formulas Tell Us About a Compound

Let's take two chemical compounds, acetylene and benzene, which share the same ratios and same empirical formula: CH, which is one hydrogen atom per one carbon atom. Compounds will share an empirical formula when the chemical formula of one is a multiple of the other. In this case the chemical formula for acetylene is C2H2 while benzene's is C6H6.

Although they have the same basic makeup, one molecule of benzyne has three times the number of atoms and three times the molar mass as an acetylene molecule. They are both classified as hydro-carbons but have different uses.

If chemists or industries were to assume that materials like acetylene and benzyne were actually the same compound based solely on their empirical formula, it could lead to flaws in the products they attempt to make, or even worse, it could lead to serious danger when a welder uses a highly toxic compound as fuel. Categorizing all these known chemicals correctly can literally save lives.

Chemists were first able to find the molecular formula of compounds using the empirical formula as a basis. The number of atoms which make up a molecular formula are always whole numbers which we multiply by the number ratio of the original empirical formula. The smallest number this factor can be is one.

To find the multiplication factor of a certain compound, scientists calculated the mole value, or average mass of an individual molecule. Going back to our examples of acetylene and benzene, laboratories were at some point able to calculate the mass values of those individual molecules, from which data they were able to obtain molecular formulas. Chemicals with entirely different atomic makeup may also share same factor.

Since we already know the molecular formula and the empirical formula for these compounds, we can work backwards in order to find the whole number by which our original formula is multiplied. The solutions look like this: