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NCERT Solutions for Class 9th Science Chapter 9 .8 Formula Unit Mass of Ionic Compounds — Pause and Ponder

Book page 1809 Updated on2026-09-08

Q23.
Find the formula unit mass of potassium chloride (KCl). Atomic mass — K = 39 u; Cl = 35.5 u.
Answer

74.5 u.

KCl contains: 1 K, 1 Cl
Formula unit mass = (39 u × 1) + (35.5 u × 1)
= 74.5 u
Why is it called formula unit mass and not molecular mass? Because potassium chloride does not exist as molecules. Its solid is a 3-D lattice in which every K⁺ is surrounded by Cl⁻ ions and every Cl⁻ by K⁺ ions — you cannot point to one K⁺ and one Cl⁻ and say “that pair is a molecule”. KCl simply records the simplest whole-number ratio of ions, 1 : 1, and that ratio is the formula unit.
Tip: 35.5 u is not a mistake. Chlorine occurs as two isotopes, of mass 35 u and 37 u, in roughly a 3 : 1 ratio, and 35.5 u is the weighted average you met in Chapter 8.
Q24.
Find the formula unit mass of magnesium hydroxide, Mg(OH)2. Atomic mass — Mg = 24 u; O = 16 u; H = 1 u.
Answer

58 u.

Mg(OH)₂ contains: 1 Mg, and 2 hydroxide groups — so 2 O and 2 H
Formula unit mass = (24 u × 1) + {(16 u × 1) + (1 u × 1)} × 2
= 24 u + (17 u × 2)
= 24 u + 34 u
= 58 u
Why the bracket matters: the subscript 2 sits outside the bracket, so it multiplies everything inside — both the oxygen and the hydrogen. Ignore the bracket and you would write 24 + 16 + 2 = 42 u, which is wrong by 16 u. The bracket is there because there are two complete hydroxide ions bound to one Mg²⁺ ion.
Check it yourself: apply the same rule to calcium nitrate in Example 9.7 — Ca(NO₃)₂ = 40 + {14 + (16 × 3)} × 2 = 40 + 124 = 164 u ✓
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