Empirical & Molecular Formula | Basic Concepts of Chemistry for JEE & NEET

Empirical & Molecular Formula

Essential Chemistry Notes & Practice for JEE (Main/Advanced) & NEET

1. Definitions

Empirical Formula (EF): Represents the simplest whole-number ratio of various atoms present in a compound. (e.g., Glucose $CH_2O$).

Molecular Formula (MF): Represents the actual number of atoms of various elements present in one molecule of the compound. (e.g., Glucose $C_6H_{12}O_6$).

2. Relationship between Empirical & Molecular Formula

The Molecular Formula is a simple whole-number multiple of the Empirical Formula.

$$ \text{Molecular Formula} = n \times (\text{Empirical Formula}) $$

Where $n$ is a positive integer (1, 2, 3...) calculated as:

$$ n = \frac{\text{Molecular Mass}}{\text{Empirical Formula Mass}} $$

Note: If Vapour Density (VD) is given, remember that $\text{Molecular Mass} = 2 \times VD$.

3. Steps to Determine Empirical Formula

When given the percentage composition of elements by mass:

1. Step 1: Assume a 100g sample. The percentage becomes mass in grams.
2. Step 2: Calculate moles of each element ($ \text{Moles} = \frac{\text{Mass}}{\text{Atomic Mass}} $).
3. Step 3: Divide all mole values by the smallest mole value obtained to get the simplest ratio.
4. Step 4: If ratios are not whole numbers (e.g., 1.5), multiply all by a suitable integer (e.g., 2) to convert them to whole numbers.

Example:

A compound contains 80% Carbon and 20% Hydrogen.

Element	% Mass	Atomic Mass	Relative Moles	Simplest Ratio
C	80	12	$80/12 = 6.66$	$6.66/6.66 = 1$
H	20	1	$20/1 = 20$	$20/6.66 \approx 3$

Empirical Formula: $CH_3$

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