## Empirical and molecular formula

The empirical formula is the simplest expression to represent a chemical compound. It tells us the elements that are present and the minimum proportion in whole numbers between their atoms. This formula is also known as “minimal formula” represented by “fe”.

For example: Bioprofe | Empirical and molecular formula | 01

To obtain it, it is necessary to know the molecular mass of each chemical element.

For example:

• Calculate the empirical formula of a substance whose centesimal composition is: 0.8% H; 36.5% Na; 24.6% of P and 38.1% of O.

Taking into account the molecular mass of each substance, the number of relative atoms of each chemical element is calculated: Bioprofe | Empirical and molecular formula | 02

If we look at which is the smallest of all and we reduce them to unity, we have: Bioprofe | Empirical and molecular formula | 03

Therefore the empirical formula of our product is Na2HPO3.

The molecular formula is the real formula of the molecule and it is formed by the symbols which are the chemical elements and some subscripts that indicate the number of atoms which participate in the formation of the molecule. It is represented by “fm”.

For example: Bioprofe | Empirical and molecular formula | 04

When we want to calculate molecular formula (fm), in addition to finding the empirical formula (fe) and the molecular weight of it (PMfe), we need to know the molecular weight of the final compound (PMc) and in this way we can apply the following equations and get the molecular formula. Bioprofe | Empirical and molecular formula | 05

For example:

• Dibutyl succinate is a repellent used in houses for insects. Its composition is 62.58% Carbon, 9.63% Hydrogen and 27.79% Oxygen. If your experimentally determined molecular weight is 239g / mol, calculate the molecular formula.

First of all we have to calculate the empirical formula by obtaining the number of atoms of each chemical element: Bioprofe | Empirical and molecular formula | 06

We reduce all to the unity and we obtain the number of atoms of each element: Bioprofe | Empirical and molecular formula | 07

The empirical formula of tope compound is C3H5,5O1. We round the subscripts multiplying all the elements by 2 and the empirical formula finally remains: C6H11O2

To obtain the molecular formula we have to relate the molecular weight of this (PMc) with the molecular weight of the empirical formula (PMfe). Bioprofe | Empirical and molecular formula | 08

With the aforementioned equation we relate the two molecular weights: Bioprofe | Empirical and molecular formula | 09

Our molecular formula is: C12H22O4.

Once we have determined the formula of the molecule we can study its structure, which is the arrangement or distribution of the different atoms joined together to give rise to the final molecule.

We must keep in mind that molecules have a spatial disposition, they are three-dimensional structures and for their representation we must resort to molecular models, where the most known are:

• Fisher projection. It owes its name to the German chemist Hermann Emil Fischer and in this projection the molecule is drawn in the form of a cross where the substituys that go to the bottom of the plane are placed in the vertical line and the groups that come out towards us are placed in the horizontal line. The point of intersection between both lines represents the Carbon atom.In each molecule when a Carbon atom is attached to four different substituents it is assigned the name of “Chiral or Asymmetric Carbon” and each Chiral Carbon gives rise to two isomeric specular molecules or enantiomers. Bioprofe | Empirical and molecular formula | 10

This projection results in two spatially distinct configurations, which are the “mirror images”. They are symmetrical images with respect to a plane but they are not superimposable to each other, as are our right and left hands.

For example we can study the case of GLUCOSE whose molecule also has 6 carbon atoms and the four central atoms are Chiral Carbon. Bioprofe | Empirical and molecular formula | 11

• Newman projection. The representation is obtained by looking at the molecule along the carbon-carbon axis, where the carbon closest to the viewer is drawn by a point from which the three bonds that join each substituent depart, while the carbon behind it is reflected with a circle from which its three substituents come out. Bioprofe | Empirical and molecular formula | 13

• Haworth projection. It owes its name thanks to the English chemist Walter Norman Haworth and this is the most used to represent carbohydrates, with pentagonal or hexagonal chemical structure, with a three-dimensional perspective.

The most used carbohydrates to visualize with this projection are among others glucose, galactose or fructose. Bioprofe | Empirical and molecular formula | 14

Finally, we must keep in mind that atoms and molecules can be represented in a three-dimensional way for a better understanding of their structure.

If we compare molecules mentioned above in several projections already studied, we can see the great change they have when we see them in their three-dimensional structure. Therefore, to really understand the structure of a molecule, it is best to observe it from all angles and with all possible projections.

MOLÉCULA NEWMAN PROJECTION THREE-DIMENSIONAL STRUCTURE
Ethane

(CH3-CH3) Bioprofe | Empirical and molecular formula | 15

2-Cloropropane
(CH3-CH2Cl-CH2-CH3) Bioprofe | Empirical and molecular formula | 17 Bioprofe | Empirical and molecular formula | 18