Molecular Formula

The molecular formula specifies the actual number of atoms of each element in a molecule.

The conventional form for writing a molecular formula is to write the symbol for each element followed by a subscript indicating the actual number of those atoms present in a molecule. When only one atom of an element is present, the subscript is omitted. For example, the molecular formula for water, H₂O, specifies that there are two hydrogen atoms and one oxygen atom present in each molecule of water.

It is important to remember that the molecular formula—in contrast to the simpler empirical formula that specifies only the relative number of atoms or moles present in a compound—identifies the actual number of atoms present in a molecule.

For example, the molecular formula for glucose (a sugar important in many biological reactions), C₆H₁₂O₆ specifies that in each molecule of glucose there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. In contrast, the empirical formula for glucose, CH₂O, specifies only that there are two hydrogen atoms for every carbon atom and one oxygen atom for every carbon atom in one molecule of glucose. If dealing with moles of glucose, the empirical formula for glucose, CH₂O, specifies only that there are two moles of hydrogen atoms for every mole of carbon atoms and one mole of oxygen atoms for every mole of carbon atoms in a mole of glucose.

More information is required to construct a molecular formula than is required to obtain the empirical formula of a substance. The empirical formula can be obtained from the elemental analysis of a substance. To obtain the molecular formula, the total molecular mass must be determined experimentally. The molecular formula is then determined from both the empirical formula and the molecular mass of a substance.

The molecular formula of a compound is always an integer multiple (e.g., 1, 2, 3,...) of the empirical formula. If the empirical formula of a compound is known, the molecular formula can be determined by the experimental determination of the molecular weight of the compound.

There are two steps to determining the molecular formula once the molecular weight of a compound has been determined experimentally.

The first step is to divide the experimentally determined molecular weight of the compound by the molecular weight of the empirical formula in order to determine the integer multiple that represents the number of empirical formula units in the molecular formula. In the second step, the molecular formula is obtained by multiplying the subscripts of the empirical formula by the integral multiple of empirical formula units.

For example, there are many carbohydrates or saccharides that have the empirical formula CH₂O and that have a molecular formula that is an integer multiple of CH₂O so that they can, as a group be generally described by the formula (CH₂O)_n, where n is an integer representing the number of empirical formula units in the molecular formula of the carbohydrate.

If the molecular weight of a carbohydrate (simple sugars) with an empirical formula of CH₂O is experimentally determined by combustion analysis to be 180 g/mole, an integer multiple of six (6) is obtained by dividing the experimentally determined molecular weight of 180 g/mole by 30g/mole (the theoretical weight of the empirical formula unit). This means that there are 6 empirical formula units in the molecular formula. When the subscripts of the empirical formula are then multiplied by the integer multiple of six (6) the result yields a molecular formula of glucose (C₆H₁₂O₆).

In converse, a molecular formula may be reducible to a simple or empirical formula if all its subscripts are divisible by a common denominator. For example, nicotine has a molecular formula of C₁₀H₁₄N₂ that can be divided to obtain the empirical formula of ₅H₇N.

Some experimental methods can be used to determine the molecular formula (i.e., the actual number of atoms that exist in a molecule). X-ray determination can, for example, allow the actual determination of the positions of atoms in some solids and thus allow direct evidence of the molecular formula.

There are compounds that have the same molecular and empirical formula. For example, methane has both an empirical formula and a molecular formula of CH₄. The simplest whole number ratio of hydrogens to carbon atoms is four hydrogen atoms to every carbon atom. Because CH₄ is also the molecular formula this specifies that in a molecule of methane, four hydrogen atoms are bonded to a single carbon atom.

The empirical formula for water, H₂O, is also the correct molecular formula for water. The molecular formula for water specifies that two hydrogen atoms are bound to a single oxygen atom.

Compounds may also share an empirical formula but dramatically differ in their molecular formulae. For example, acetylene and benzene both have an empirical formula of CH. Acetylene has a molecular formula of C₂H₂ while benzene, has a molecular formula of C₆H₆.

Some materials do not actually exist as isolated molecules so it is technically impossible to give a molecular formula for such substances. For example, table salt (NaCl) does not exist as a molecular substance but rather as an ionic bonded crystal lattice. Regardless, the terms molecular mass and molecular formula are often casually (but now incorrectly) applied to these ionic substances. The reason for this confusion in terminology is that the term molecule originally was defined to be any aggregate of atoms bonded (by either ionic or covalent bonds) together in close enough order to be considered as a discrete physical structure. In this regard, a molecule was considered to be a unit of—or the smallest particle of—a compound that retained the chemical properties of the substance.

Ionic compounds, however, are composed of ions, not covalently bonded atoms. For ionic compounds formula mass should be used instead of molecular mass and empirical formula, simplest formula or formula unit should be used instead molecular formula.

Substances can be molecular (linked together by covalent bonds) or ionic (associated by ionic electrical attraction). Molecular substances are described by their molecular formula (e.g., H₂O or CH₄. Ionic substances are described by the formula unit (e.g., NaCl or MgF₂). When dealing with ionic compounds, the smallest whole-number subscripts are always used.

The molecular formula does not provide information on which atoms are arranged in a molecule. The structural formula is needed to determine the actual arrangement of the atoms in a molecule. Methyl alcohol, for example, consists of a carbon atom that has three hydrogen atoms and one oxygen atom bonded to it. The oxygen atom is, in turn, bonded to a fourth hydrogen atom. The molecular formula is CH₄O does not convey this specificity of arrangement as does the structural formula, CH₃OH.

Like the molecular formula, the structural formula of a substance gives the exact number of atoms of each element per molecule. In addition, however the structural formula depicts how the atoms are bonded to each other. As a result, the structural formula is essential for the determination of molecular shape and the properties associated with particular molecular geometric arrangements.

Just as there are substances with the same empirical formula that differ in their molecular formula, there are substances that have the same empirical and molecular formula. The only way to differentiate these substances is by determining the structural formula for each substance. For example, Cis dibromoethene and Trans dibromoethene have the same empirical formula (CHBr) and the same molecular formula (C₂H₂Br₂) and must be distinguished by a unique structural formula.