What is the formula of following coordination compound:
Potassium trioxalatoaluminate(III)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(K_3[Al(C_2O_4)_3]\)
To write a formula from the name of a coordination compound, read the name part by part. The cation is named first, then the complex ion, whose ligands come before the metal. A name ending in “-ate” shows that the complex is an anion, and the Roman numeral gives the oxidation state of the metal.
“Potassium”: the cation is \( \ce{K+} \).
“Trioxalato”: three oxalate ligands, \( \ce{C2O4^2-} \).
“Aluminate(III)”: one aluminium atom in the +3 state, in an anionic complex.
Charge on the complex ion:
\( \displaystyle (+3) + 3(-2) = -3 \Rightarrow \ce{[Al(C2O4)3]^3-} \)
Three \( \ce{K+} \) ions balance this charge, so the formula is \( \ce{K3[Al(C2O4)3]} \).
Why the other options are incorrect
Option (2) \(K[Al_2(C_2O_4)_3]\) — Incorrect. It has two Al atoms, which the name would show as “di”; also \( 2(+3) + 3(-2) = 0 \), so the bracket would be neutral and need no \( \ce{K+} \).
Option (3) \(K_3[Al_2(C_2O_4)_3]\) — Incorrect. The bracket is again neutral, so three \( \ce{K+} \) ions would leave the compound with a charge of +3.
Option (4) \(K[Al(C_2O_4)_3]\) — Incorrect. The complex ion has a charge of −3, and one \( \ce{K+} \) cannot balance it.
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “Example 5.2: Write the formulas for the following coordination compounds: … (c) potassium trioxalatoaluminate(III) …
Solution: … (c) K3[Al(C2O4)3]”