Empirical vs Molecular Formulas
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.
Molecular formula is the formula 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. And the numbers of atoms are written as subscripts. Some molecular formulas are neutral (no charge), but if there is a charge that can be shown in 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, molecular mass can be calculated. 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. Further, the oxidation numbers of each atom, how they are going to react in a reaction, and the resulted products can be predicted using molecular formulas. However, from the molecular formula only, we cannot predict the exact molecular arrangement. Because there can be several structural formulas for a single molecular formula. These are known as 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.
Empirical formula is the simplest form of formula 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. Further, Ca3(PO4)2 is also a empirical formula. In an ionic compound, formula can be easily written by interchanging the charges of each ion, and that automatically gives the number from each ion in the molecule. Also, empirical formulas are written for macromolecules. When writing empirical formulas for polymers, the repeating unit is written, and then “n” is used to say that there can be n number of repeating units in the polymer. Empirical formula cannot be used to find the mass, structure or isomers for a molecule, but it can be useful for analytical purposes.
What is the difference between Molecular Formula and Empirical Formula?
- Molecular formula gives the exact number of each atom in a molecule. But the empirical formula only gives the simplest ratio of atoms.
- Unlike molecular formula, from the empirical formula, much detail about the molecule cannot be derived.