# Molecular Formula

The molecular formula is the expression of the number of atoms of each element in one molecule of a compound.

The molecular formula definition is the formula showing the actual number of each atom in a molecule.

When the molar mass value is known, the Molecular Formula is calculated by the empirical formula.

The molecular formula is often same as empirical formula or an exact multiple of it.

Example 1

Caffeine has the following composition: 49.48% of carbon, 5.19% of hydrogen, 16.48% of oxygen and 28.85% of nitrogen. The molecular weight is 194.19 g/mol. Find out the molecular and empirical formula.

Solution

Step 1

Multiply percent composition with the molecular weight

Carbon – 194.19 x 0.4948 = 96.0852

Hydrogen – 194.19 x 0.0519 = 10.07846

Oxygen –  194.19 x 0.1648 = 32.0025

Nitrogen – 194.19 x 0.2885 = 56.0238

Step 2

Divide each value by the atomic weight

Carbon : 96.0852 / 12.011 = 7.9997

Hydrogen : 10.07846 / 1.008 = 9.998

Oxygen : 32.0025 / 15.9994 = 2.000

Nitrogen : 56.0238 / 14.0067 = 3.9997

Step 3

Round off the values to closest whole number

8: Carbon

10: Hydrogen

2: Oxygen

4: Nitrogen

Hence, the molecular formula is C8H10N4O2.

Step 4

Since 2 is the common factor among 8, 10, 4 and 2.

The empirical formula is C4H5N2O.

Example 2

A compound contains 86.88% of carbon and 13.12% of hydrogen, and the molecular weight is 345. What will be the empirical and molecular formula of the compound?

Solution

Step 1

Carbon : 345 x 0.8688 = 299.736

Hydrogen : 345 x 0.1312 = 45.264

Step 2

Carbon : 299.736 / 12.011 = 24.995

Hydrogen : 45.264 / 1.008 = 44.91

Step 3

Molecular formula is C25H45

Step 4

Since 5 is the common factor, the empirical formula is C5H9

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