Question

4. Derive the molecular formulae of the following compounds stepwise and give their atomicity.

i. ammonium sulphate

Answer 1

Derive the molecular formulae of ammonium sulphate stepwise and give their atomicity:

1. Write the symbols of the elements or the ions forming the compound.

${\mathrm{NH}}_4$ ${\mathrm{SO}}_4$

2. Write the valency of each as a superscript to the top right. Ignore the + and - signs of ions.

${{\mathrm{NH}}_4}^1$ ${{\mathrm{SO}}_4}^2$

3. Interchange the valencies and bring them down diagonally.

4. Write the interchanged valencies as subscripts. Place polyatomic ions in parentheses if the subscript is greater than 1.

${\left({\mathrm{NH}}_4\right)}_2{\left({\mathrm{SO}}_4\right)}_1$

5. Ignore the number 1. Reduce the numbers to the lowest ratio. Write the chemical formula.

${\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$

• Atomicity: Atomicity is the total number of atoms in a molecule. Molecules may be monoatomic, diatomic, triatomic or polyatomic depending on the number of atoms. Thus, the atomicity of (NH₄)₂SO₄ is 15.