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What is a molecular formula?

A molecular formula is a way of representing chemical substances in which the exact atomic composition of the chemical is shown. It is a formula that indicates the types of atoms and the number of them that make up the molecule of a pure substance.

In the molecular formula, the different types of atoms are represented by their chemical symbols, using subscripts to indicate the number of times each atom is repeated. In all cases, the subscript 1 is omitted.

What substances have a molecular formula and what substances do not?

It is very important to mention that, as its name indicates, molecular formulas only apply to molecular compounds, that is, to those that are made up of discrete units, called molecules, in which the intramolecular forces that hold the atoms together (ie, covalent bonds) are much stronger than the cohesive forces that hold molecules together.

In this sense, molecular formulas do not apply to ionic compounds , since these are not formed by molecules but by ions. In ionic compounds, each cation is simultaneously bound to several anions and not to one in particular. Due to the nature of the ionic bond, there is no discrete unit made up of an anion and a cation in these compounds. However, it is common for people to refer to the units of these compounds as molecules, and their empirical formulas as molecular formulas, despite this being a considerable misconception from a chemical point of view.

In other words, saying that the molecular formula of sodium chloride is NaCl is a mistake , since sodium chloride is an ionic compound, not a molecular compound. Having said this, it should be noted that at a practical level the use of one or the other formula is exactly the same, so making this conceptual error is inconsequential from a practical point of view (never from a theoretical one!).

On the other hand, the molecular formulas also do not apply to covalent solids, that is, those that are formed by a one-, two-, or three-dimensional network of atoms joined together by covalent bonds. In these cases, there is not a single repeating molecule in the compound, but each crystal is itself a large molecule whose total number of atoms varies. In these cases, another type of formula called an empirical formula is used .

Utility of the molecular formula

Molecular formulas are of great importance, since they allow you to quickly see the elemental composition of a molecular compound, making it very quick and easy to calculate variables such as molecular weight and, therefore, the molar mass of the substance. Most of the stoichiometric calculations that chemists do on a daily basis are carried out by means of molar masses.

For example, the molecular formula of carbon dioxide is CO 2 , so its molecular weight corresponds to the sum of the weight of one carbon atom (12,011) and two oxygen atoms (15,999 each):

What is a molecular formula - molecular weight

In addition to this, molecular formulas also allow establishing stoichiometric relationships between the elements that make up a substance. Thus, in the case of the water molecule, whose molecular formula is H 2 O, we can see that there are 2 hydrogen atoms for every oxygen atom.

Finally, molecular formulas allow us to determine when two chemical compounds are isomers of each other. Isomerism is the relationship between two chemical substances that are different or in some way distinguishable from one another, but share the same molecular formula.

For example, ethanol or ethyl alcohol and dimethyl ether are two different organic compounds that have very different physical and chemical properties (the former is a liquid while the latter is a gas at room temperature, for example). However, both substances share the same molecular formula, namely C 2 H 6 O, which is why they are isomers.

Limitations of the molecular formula

Molecular formulas have the disadvantage that they only show the composition of a molecule, but do not show the connectivity between the atoms that make it up. In other words, it does not tell how or in what order the atoms are bonded, but only which atoms are present.

This limits its use to the applications mentioned in the previous section, but it is not particularly useful for understanding how or why molecules are formed, nor does it allow one to understand and compare their properties. There are other formulas that some people also refer to as molecular formulas and that provide much more information. Such is the case of semi-developed formulas, structural formulas, Lewis structures and others. However, none of these is really a proper molecular formula.

Molecular Formula vs. Empirical Formula

A formula that is related to the molecular formula but not the same thing is the empirical formula. This represents the composition of a chemical substance (be it ionic or molecular), showing only the elements that compose it and the minimum relationship of integers that can be written between all its atoms.

Empirical formulas turn out to be a simplified version of the molecular formula. In other words, the molecular formula is always an integer multiple of the empirical formula. For example, hydrogen peroxide is a compound with the molecular formula H 2 O 2 . This 2:2 ratio between hydrogen and oxygen atoms can be represented by simpler whole numbers, namely 1:1, so the empirical formula for hydrogen peroxide is HO.

Molecular formula versus semi-developed formulas

As mentioned before, molecular formulas do not show the connectivity between the atoms in a molecule. For this, there are developed structural formulas or Lewis structures. However, there is a type of formulas that have an intermediate character between the molecular formula and the structural one called semi-developed formula.

In these formulas, the atoms that make up a molecule are grouped according to their connectivity, and the groups are often written in the order in which they are bonded. These formulas are easy to recognize because they sometimes have parentheses and can have the same element multiple times in different parts of the formula.

For example, ethanol can be represented as C 2 H 5 OH, where emphasis is placed on the fact that there is a first group of atoms (C 2 H 5 -) in which carbon and hydrogen are bonded together. , and then there’s another group of atoms (the OH) attached to it.

Examples of molecular formulas

The table below shows some examples of molecular formulas for common compounds.

Name Molecular formula   Name Molecular formula Water H2O _ _   Glucose C 6 H 12 O 6 dinitrogen pentoxide No 2 O 5   Ammonia NH3 _ Aluminum oxide At 2 or 3   Butane C 4 H 10 Acetic acid C2H4O2 _ _ _ _ _   Benzene C6H6 _ _ _ sulfur dioxide OS 3   phosphoric acid H 3 PO 4


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