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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?
Formula Unit Mass
The sum of the atomic masses of all atoms in one formula unit of an ionic compound. Sodium oxide Na2O equals 62 u
- 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 Na 2 O = (23× 2) + (16× 1) = 62 u .
Example 9.7 — Formula unit mass of calcium
Atomic mass: Ca = 40 u, N = 14 u, O = 16 u. Formula unit mass of Ca(NO 3 ) 2 = 40 + 14 + (16× 3)× 2 = 164 u .
⏸▶ Pause and Ponder
- 23. KCl = 39 + 35.5 = 74.5 u .
- 24. Mg(OH)₂ = 24 + (16+1)× 2 = 58 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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✅ Test Yourself
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What is a formula unit?
Show Answer
The simplest whole number ratio of ions in an ionic compound. -
What is formula unit mass?
Show Answer
The mass of one formula unit of an ionic compound. -
Find the formula unit mass of Na₂O (Na = 23 u, O = 16 u).
Show Answer
(23× 2) + (16× 1) = 62 u. -
Why do we say “formula unit mass” and not “molecular mass” for NaCl?
Show Answer
Because ionic compounds do not exist as separate molecules.
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.
