Empirical Formula & 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 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 × Empirical Formula

Table of content:

• What is molecular formula
• What is empirical formula
• Recommended Videos
• Empirical formula Vs Molecular formula
• Example
• Solved examples
• Frequently Asked Questions – FAQs

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.

Recommended Videos

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 × empirical formula where n is a whole number

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

Solved Examples

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) = 13.81u

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

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

n = molecular formula/empirical formula

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 COCl₂ and its 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 the same as molecular mass as n=1, this means molecular formula is COCl₂.

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 C₅H₁₀

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