Lecture 4 – Empirical & Molecular Formula Calculations

Empirical Formula:

Empirical formula is defined as the simplest formula of the compound which shows the relative number (s) of atoms of different elements present in the compound.

Determining Empirical Formula:

For determining Empirical Formula, consider the following example:

Molecular Formula:

Molecular formula of a compound shows the actual number of elements present in that compound.

For example; the empirical formula of Ethane is CH₃ but its molecular formula is C₂H₆.

Determining Molecular Formula:

For determining Molecular Formula, consider the following example:

Example: A compound contains 80% Carbon and 20% Hydrogen. Its M_r is 30. Determine its empirical formula and its molecular formula.

Step 1: Determine the Empirical formula of the compound.

Step 2: Calculate the empirical formula mass of the compound

A_r of C = 12

A_r of H = 1

empirical formula mass of CH₃ = A_r of C + 3(A_r of H)

empirical formula mass of CH₃ = 12 + 3(1) = 12 + 3 = 15

Step 3: Calculate n

M_r of the compound = n x empirical formula mass of the compound

Step 4: Forming the molecular formula

Molecular formula = n x Empirical Formula

Molecular formula = 2 x CH₃

Molecular formula = C₂H₆