Difference Between Empirical and Molecular Formulas

The key difference between empirical and molecular formulas is that an empirical formula only gives the simplest ratio of atoms whereas a molecular formula gives the exact number of each atom in a molecule.

In chemistry, we often use symbols to identify elements and molecules. Molecular formula and empirical formula are two such symbolical methods we use to represent molecules and compounds in an easy way.

1. Overview and Key Difference
2. What are Empirical Formulas
3. What are Molecular Formulas
4. Side by Side Comparison – Empirical vs Molecular Formulas in Tabular Form
5. Summary

What are Empirical Formulas?

Empirical formulas are the simplest form of formulas that we can write for a molecule. It shows the type of atoms in the molecule, but it doesn’t give the real number of each atom. Rather, it gives the simplest integer ratio of each atom of the molecule.

For example, C₆H₁₂O₆ is the molecular formula of glucose, and CH₂O is its empirical formula. Mostly, we give empirical formulas for ionic compounds, which are in the crystalline form. For instance, we cannot say the exact number of Na and Cl in a NaCl crystal. So we just write the empirical formula denoting the ratio of connected atoms.

Video 01: Empirical Formula

Further, Ca₃(PO₄)₂ is also an empirical formula. In an ionic compound, we can easily write the formula by interchanging the charges of each ion, and that automatically gives the number from each ion in the molecule. Also, we can write empirical formulas for macromolecules. When writing empirical formulas for polymers, we write the repeating unit, and then we use the letter “n” to say that there can be n number of repeating units in the polymer. However, we cannot use the empirical formula to find the mass, structure or isomers of a molecule, but it is useful for analytical purposes.

What are Molecular Formulas?

Molecular formulas are the formulas showing the type of atoms and number of each atom connected in the molecule. Therefore, it gives the correct stoichiometry of each atom. The atoms are depicted by their symbols, which are shown in the periodic table. Moreover, we should write the numbers of atoms as subscripts. Some molecular formulas are neutral (no charge), but if there is a charge, we can show it on the right side of it as a superscript.

We normally use the molecular formula in the chemical reactions, or when documenting any chemical details. By just looking at the molecular formula, we can get a lot of information about the molecule. For example, we can calculate the molecular mass. Also, if it is an ionic compound, we can predict what are the ions and how many of them will be released when it is dissolved in water.

Video 02: Comparison of Molecular and Empirical Formulas

Furthermore, we can predict the oxidation numbers of each atom, how they are going to react in a reaction, and the resulted products using molecular formulas. However, from the molecular formula only, we cannot predict the exact molecular arrangement. Because sometimes there are several structural formulas for a single molecular formula. These are the “isomers”. Isomers have the same molecular formula but can differ from the connectivity of atoms (constitutional isomers) or the spatial arrangement of atoms (stereoisomers). So, by looking at the molecular formula, we can write all the possible isomers for a molecule.

What is the Difference Between Empirical and Molecular Formulas?

Empirical formulas are the simplest form of formulas that we can write for a molecule while molecular formulas are the formulas showing the type of atoms and number of each atom connected in the molecule. Hence, the key difference between empirical and molecular formulas is that empirical formula only gives the simplest ratio of atom whereas molecular formula gives the exact number of each atom in a molecule. Furthermore, we cannot calculate the exact molecular mass using the empirical formula while we can get the exact molecular mass using the molecular formula.

As another important difference between empirical and molecular formulas, we can say that we can predict the type of atoms in ionic compounds and repeating unit in a polymer whereas we can predict the oxidation numbers of each atom, how they are going to react in a reaction, and the resulted products using molecular formulas.

Summary – Empirical vs Molecular Formulas

For a certain molecule or a macromolecule, we can write an empirical formula. However, the molecular formula is the more detailed form. The key difference between empirical and molecular formulas is that an empirical formula only gives the simplest ratio of atom whereas a molecular formula gives the exact number of each atom in a molecule.

Reference:

1. “Empirical Formula 2.” Transition Metals. Available here
2. Libretexts. “6.9: Calculating Molecular Formulas for Compounds.” Chemistry LibreTexts, National Science Foundation, 20 May 2018. Available here