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.

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Practise This Question

Solubilitity of alkynes in H2O is more than their corresponding alkanes and alkenes - True or False?