# Empirical And Molecular Formula

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 type of formulas in chemistry: molecular formula and empirical formula.

### 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.

### Empirical formula

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

#### Example: Glucose molecular formula vs empirical formula

Let’s take an 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 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 × empirical formula where n is a whole number

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

#### Question

The empirical formula of Boron Hydride is BH3. Calculate 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) = 13.81u

But, the measured molecular mass for Boron is given as 27.66u

By using the expression, Molecular formula = n × empirical formula

n = molecular formula/empirical formula = $\frac {27.66}{13.81}$ = 2

Putting value of n = 2 in the empirical formula we get molecular formula as

Molecular formula = 2(BH3) = B2H6.

#### Question

The empirical formula of a compound is COCl2 and is molecular mass is 90.00u.Find out the molecular formula of that compound.

#### Solution

COCl2 = C + O + 2(Cl) = 12 + 16 + 2(35.5) = 99 u

Empirical formula is same as molecular mass as n=1, this means molecular formula is COCl2.