Determination of Empirical and Molecular Formula

The symbolic representation of a molecule is known as the molecular formula. A molecular formula also known as the chemical formula employs symbols to denote the molecule of an element or of a compound. A molecule of an element may contain one or more atoms of it. For example; a molecule of elements hydrogen, oxygen, nitrogen, chlorine etc. contains two atoms and are written as H2, O2, N2, Cl2. 

The empirical formula is the whole number of atoms which is present in the chemical compound and is the simplest formula. For example; the empirical formula of calcium chloride is CaCl2, silicon dioxide is SiO2 etc. Formaldehyde, glucose, acetic acid, ribose are some of the other examples of the empirical formula.

Determination of Empirical Formula

The initial steps involved in determining the empirical formula are as follows;

• The masses of the constituents of the elements are measured.

• Use molar masses of the elements in order to convert the element’s mass into a number of moles.

• Convert the number of elements into the whole number for determining or deriving the substance’s empirical formula. 

The final steps involved in determining the empirical formula are as follows;

• Firstly, find out the number of moles of individual elements as per the weight given and their mass as described in initial steps.

• Make the formula of the compound and decide the smallest subscript mole from both of them.

• Divide the moles of both the elements by the smallest subscript in order to get the whole number and the empirical formula of the chemical compound.

For example;

Consider the sample compound containing 1.51 gram N (Nitrogen) and 0.324 gram H (Hydrogen). The number of moles of each of them is represented as;

For Nitrogen, 1.51×(1 mol N/14.01 g N) = 0.1078 moles of nitrogen N.

For hydrogen, 0.324×(1 mol H/1.008 g H) = 0.321

The compound now can be represented as N0.1078H0.321 but for the whole number, the moles of the compound are divided by the subscript having a small number i.e. 0.1078 and it can be represented as;

N0.1078/0.1078H0.321/0.178 = NH3 (ammonia compound).

So the empirical formula of ammonia is NH3.

Determination of Molecular Formula

The steps in determining the molecular formula are as follows;

• The average sum of atomic masses of the atoms is determined which is known as empirical formula mass or mass of empirical formula. 

• Find out the molar or molecular mass of the elements.

• Divide this molar or molecular mass of the elements by the empirical formula mass for determining the empirical formula numbers units per mole or molecule (n) which can be represented as

n = moles or molecular mass of element (g/mol or amu)/ empirical formula mass (g/mol or amu)

• The molecular formula is finally determined by multiplying the subscript of the each element in empirical formula by the formula molecules/units (n) i.e. (AyBz)n= AynBxn. 

Significance of Molecular Formula

The molecular formula of a chemical compound has quantitative significance and it represents;

• The molecular mass as well as the molecule of the chemical compound.

• The different atomic numbers of the compound present in one molecule.

• The ratios of compound’s molecular masses of an element.

Conclusion

The molecular formula determines the ratio in which the different atoms are united to form the molecule of the chemical compound. The molecule containing different atoms united in a certain fixed ratio is represented by placing symbols of the elements present in it side by side indicating their numbers written in subscript. For example; 2H2O represents two molecules of water i.e. dihydrogen oxide, each molecule containing two atoms of hydrogen and one atom of oxygen. The empirical formula represents the whole number of atoms of the chemical compound.