molecular formula

The molecular formula of a substance is the chemical formula that gives the actual number of atoms of each element in the substance.

A molecular formula is the same as or a multiple of the empirical formula. For example, the empirical of carbon dioxide is CO₂ and the molecular formula is also CO₂. Whereas, the empirical formula of ethane is CH₃ while the molecular formula of ethane is C₂H₆.

Example
Given that the empirical formula of benzene is CH and its relative molecular mass is 78. Find the molecular formula of benzene. [Relative Atomic Mass: Carbon: 12; Hydrogen: 1]

Solution
Let's say the molecular formula of benzene is CnHn.

The relative molecular mass of CnHn
= n(12) + n(1)
= 13n

13n = 78
n = 78/13

= 6
Therefore, the molecular formula of benzene = C6H6

Example 2

What is the mass of metal X that can combine with 14.4g of oxygen to form X oxide with molecular formula X₂O₃. (RAM: O = 16; X = 56 )

Solution

Number of mole of oxygen

=14.4/16

=0.9 mol

From the molecular formula, we learn that the ratio of element X to oxygen

X:O = 2:3

Therefore, the number of mole of

X=0.9×23=0.6 mol

Number of mole,

n = mass/Molar mass

56 x 0.6= Mass

mass=33.6g

The mass of element X = 33.6g