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💡 Mass of a formula unit
Ionic compounds have no molecules — only endless lattices. So we add up the atoms in one formula unit instead, and call it the formula unit mass.
What is formula unit mass?
- A formula unit is the simplest whole-number ratio of ions in an ionic compound.
- Its mass is the formula unit mass .
- Found by adding the atomic masses of all atoms in one formula unit.
Example 9.6 — Formula unit mass of sodium
Atomic mass: Na = 23 u, O = 16 u. Formula unit mass of \( \text{Na}_2\text{O} = (23\times 2) + (16\times 1) = 62\ \text{u} \).
Example 9.7 — Formula unit mass of calcium
Atomic mass: Ca = 40 u, N = 14 u, O = 16 u. Formula unit mass of \( \text{Ca(NO}_3)_2 = 40 + \{14 + (16\times 3)\}\times 2 = 164\ \text{u} \).
⏸▶ Pause and Ponder
- 23. KCl \(= 39 + 35.5 = \mathbf{74.5}\ \text{u}\).
- 24. Mg(OH)₂ \(= 24 + (16+1)\times 2 = \mathbf{58}\ \text{u}\).
NCERT Question 8 — Determine the formula unit mass
Determine the formula unit mass of the following substances. (i) Ammonium nitrate (NH₄NO₃), used as a nitrogen fertiliser, which is essential for plant growth. (ii) Phosphoric acid (H₃PO₄), used to make phosphate fertiliser and detergents. (iii) Sodium hydrogencarbonate (NaHCO₃), used to relieve acidity and helps in digestion.
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Important Definitions
- Formula unit — the simplest whole-number ratio of ions in an ionic compound.
- Formula unit mass — the sum of the atomic masses of all the atoms present in one formula unit.
