Initially, chemical formulas were obtained by determination of masses of all the elements that are combined to form a molecule and subsequently we come up with two important types of formulas in chemistry : molecular formula and empirical formula.

The empirical formula of a compound gives the simplest ratio of the number of different atoms present, whereas the molecular formula gives the actual number of each different atom present in a molecule. If the formula is simplified then it is an empirical formula. The molecular formula is commonly used and is a multiple of the empirical formula.

The general statement relating molecular formula and the empirical formula is

Molecular Formula = n x Empirical Formula

What is Molecular formula?

•        The molecular formula is the formula derived from molecules and is representative of the total number of individual atoms present in a molecule of a compound.

•        A molecular formula uses a subscript that reports the actual number of each type of atom in a molecule of the compound.

•        Molecular formulas are associated with gram molecular masses that are simple whole number multiples of the corresponding empirical formula mass.

What is Empirical formula?

•        The empirical formula is the simplest formula for a compound which is defined as the ratio of subscripts of the smallest possible whole number of the elements present in the formula. It is also known as the simplest formula.

•        An empirical formula for a compound is the formula of a substance written with the smallest integer subscript.

•        The empirical formula gives information about the ratio of numbers of atoms in the compound. The percent composition of a compound directly leads to its empirical formula. Empirical Formula Vs Molecular Formula.

Example : Glucose Molecular Formula Vs Glucose Empirical Formula

Let’s take the example of glucose. The molecular formula of glucose is C₆H₁₂O₆ and the empirical formula of glucose is CH₂O. We can derive a relation between the Molecular formula and the empirical formula of glucose.

Empirical Formula & Molecular Formula of Butane & Octane [/caption]

C₆H₁₂O₆ = 6 × CH₂O

We can derive a general expression as,

Molecular formula = n x empirical formula where n is a whole number

Sometimes, the empirical formula and molecular formula both can be the same.

Question-1 : The empirical formula of Boron Hydride is BH3. Calculate the molecular formula when the measured mass of the compound is 27.66.

Solution : 

The atomic mass is given by = B + 3(H) = 10.81 + 3(1) = 13.81 u

But, the measured molecular mass for Boron atom is given as 27.66 u.  By using the expression,

Molecular formula = n x empirical formula n

                             = molecular formula/empirical formula

                             = 27.66 / 13.81

                             = 2

Putting the value of n = 2 in the empirical formula we get molecular formula as Molecular formula = 2(BH₃) = B₂H₆.

Question-2 : The empirical formula of a compound is COCI₂ and its molecular mass is 90.00u. Find out the molecular formula of that compound.

Solution :

COCI2 = C + O + 2(CI) = 12 + 16 + 2(35.5) = 99u

Empirical formula is the same as molecular mass as n=1, this means molecular formula is COCI₂.

Question-3 : What is the molecular formula of a compound which has an empirical formula of CH₂ and a relative molecular mass of 70?

Solution :

Relative molecular mass = 70

Empirical formula mass = 12 + 2 = 14

The relative molecular mass is 5 x the relative empirical formula mass

The molecular formula is 5 x the empirical formula The molecular formula is C₅H_{10 .}

Question-4 : A compound of iron and oxygen is analysed and found to contain 69.94% iron and 30.06% oxygen. Find the empirical formula of the compound.

Solution :