What Is a Molecular Formula?

M.J. Casey

The molecular formula is a notation used to describe the composition of a substance. Most formulas are written in a way that also gives some detail of the bond structure of the material. A related notation includes empirical formulas. Some materials cannot be accurately represented by the use of molecular formulas.

The properties of matter come from the charges of protons, neutrons, and electrons of each atom.
The properties of matter come from the charges of protons, neutrons, and electrons of each atom.

Matter is made up of atoms, the smallest amount of mass that retains chemical properties when subdivided. Atoms bond in different ways to form molecules. A molecular formula represents the elemental composition of one molecule.

A hand-written molecular formula uses subscripts.
A hand-written molecular formula uses subscripts.

Water’s molecular formula is H2O. The number two indicates two hydrogen atoms in the molecule. The O appears without a number, meaning one oxygen atom. Depending on the text, the numerals may be written without subscripts. Hand-written formulas always employ the subscript.

Propane is a component of natural gas. This hydrocarbon has three carbons with linear bonds forming a straight chain. The carbon chain is surrounded by eight hydrogen molecules. The molecular formula is C3H8. In the refining process, some propane will react to form cyclopropane. Cyclopropane is a triangular molecule surrounded again by six hydrogen atoms, with a molecular formula of C3H6.

A hydrogen molecule, H2, was lost in the production of the cyclopropane. Other reactions were possible, including losing a hydrogen molecule between two of the three carbons. Propene, which has a double bond between the first and second carbons, also has a molecular formula of C3H6. While the formula of C3H8 must be propane, C3H6 is insufficient information to identify the compound.

The empirical formula is written as the smallest repeating unit of a molecule. The notation requires the subscripts in the formula be positive integers. The molecular and empirical formulas for glucose are C6H12O6 and CH2O respectively. In this case, six empirical formulas of glucose are needed to construct the molecule. Empirical formulas may be thought of as the lowest common denominator.

Molecular formulas are used to calculate the molecular weights of materials, which in turn provide the values to determine the number of molecules or ions present in a given mass of material. These calculations are needed to evaluate the results of a reaction or to prepare a solution of a known concentration. Molecular formulas are not a direct measure of density or mass, or state of being.

Macromolecules, both natural and synthetic, are difficult to characterize by their molecular formulas. These long-chain molecules often have branch points with their own long chains. Typically, the monomer, or starting chemical from which the polymer was derived, is used as a pseudo-molecular formula. Polyethylene is made by removing a hydrogen molecule from ethane to form ethane that then polymerizes. It has a repeating unit of (CH2)n, the n representing an unknown but large number.

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