Potassium tetrahydroxidozincate (II) can be represented by which of the following formula?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(K_2[Zn(OH)_4]\)
In naming a coordination compound the cation is named first and the anion next. When the complex ion is an anion, the name of the metal ends in -ate (zincate), and the oxidation state of the metal is given in Roman numerals. The ligands are named before the metal, with a prefix for their number (tetra-hydroxido = four \( \ce{OH^-} \)). The complex ion is written inside square brackets, and the counter ions stay outside.
Here the complex ion contains Zn in the +2 state and four hydroxide ions:
\( \displaystyle \text{charge} = (+2) + 4(-1) = -2 \quad \Rightarrow \quad \ce{[Zn(OH)4]^2-} \)
Two \( \ce{K+} \) ions balance the charge of \( -2 \), so the formula is \( \ce{K2[Zn(OH)4]} \).
Why the other options are incorrect
Option (1) \(K_3[Zn(OH)_4]\) — Incorrect. Three \( \ce{K+} \) ions would need a complex ion of charge −3, which means zinc in the +1 state, not zinc(II).
Option (3) \([K_2(OH)_4]Zn\) — Incorrect. This puts potassium inside the coordination sphere as the central atom; potassium is only the counter ion, and zinc is the central metal.
Option (4) \([K(OH)_4]_2Zn\) — Incorrect. This also makes potassium the central atom and zinc a counter ion; no such compound exists.
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… If the complex ion is an anion, the name of the metal ends with the suffix – ate. …”
Naming of Mononuclear Coordination Compounds – “Example 5.2: Write the formulas for the following coordination compounds: … (b) potassium tetrahydroxidozincate(II) …
Solution: … (b) K2[Zn(OH)4]”