Determining Molecular Formula from Empirical Formula

Many substances consist of Molecules. A molecule consists of a small number of atoms joined together by covalent bonds.

The Molecular Formula shows all the atoms of each element contained in one molecule.

Empirical Formula shows the simplest ratio of the different types of elements present in a molecule.

For example, Butane has Molecular Formula of C₄H₁₀ and Empirical Formula of C₂H₅.

As such, Molecular Formula is a simple multiple of the Empirical Formula:

Molecular Formula   =   n   X   Empirical Formula

where n = 1, 2, 3, etc

The molecular formula can be calculated from:

1. Empirical Formula (we have discussed how to obtain it)
2. Relative Molecular Mass, Mr of Compound

Let’s take a look at an example.

Example:

Glucose has empirical formula CH₂O and relative molecular mass of 180. Find the molecular formula of glucose.

(given: H = 1.0; C = 12.0; O = 16.0)

Suggested Answer:

Relative mass of Empirical Formula, CH₂O = 12 + 2(1) + 16 = 30

Relative molecular mass   =   n   X   relative mass of the empirical formula

thus, n = Relative molecular mass / relative mass of the empirical formula

= 180 / 30 = 6

Therefore, Molecular Formula of Glucose =   6   X   CH₂O   =   C₆H₁₂O₆

Let me know how you feel about this blogposts. Drop me a comment.

PS: Feel free to share this post with your friends. 🙂

Related Articles:

• Key Definitions & Formulae in Atoms, Molecules and Stoichiometry
• Common Errors in Calculating Empirical Formula
• Organic Chemistry: Types of Formulae
• Calculating Relative Atomic Mass from Isotopic Abundance
• Determination of Molecular Formula of Hydrocarbons using Combustion Data