Class 9

9.4.6 Deduction of Molecular Formula from the Structural Formula

This is the sixth lecture from Chapter 4: “Stoichiometry” in the new Class 9 Chemistry book (Punjab Board – PCTB). It discusses the deduction of molecular formula from the structural formula of the compounds.

Notes

Molecular formula of a compound can be deduced from its structural formula by following these steps:

  1. Write down the structural formula of the compound.
  2. Count all the number of atoms present in the structural formula.
  3. Write symbols of all the elements
  4. Write the total number of atoms of each element as subscript.
  5. Remove the subscript 1.

Write down the structural formula of sulphuric acid. Its structural formula is:

Chemical Structure of Sulphuric Acid (H2SO4)

It has 2 H, 1 S and 4 O atoms.

Its molecular formula will be H2SO4.

Write down the molecular formula of acetic acid. Its structural formula is:

Chemical Structure of Acetic Acid (CH3COOH)

It has 2 C, 4 H and 2 O atoms.

Its molecular formula will be C2H4O2.

Find out the molecular formula of phosphoric acid. Its structural formula is:

Chemical Structure of Phosphoric Acid (H3PO4)

It has 3 H, 1 P and 4 O atoms.

Its molecular formula will be H3PO4.

Determine the molecular formula of n-propyl alcohol. Its structural formula is:

Chemical Structure of n-propyl alcohol

It has 3 C, 8 H and 1 O atoms.

Its molecular formula will be C3H8O.

Write down the formula of calcium carbonate. Its structural formula is:

Chemical Structure of Calcium Carbonate (CaCO3)

It has 1 Ca, 1 C and 3 O atoms.

Its chemical formula be CaCO3.