Correct formula of the compound which gives a white precipitate with BaCl₂ solution, but not with AgNO₃ solution, is :

Solution

Chemistry ma’am · 3 steps

Only ions outside the square bracket are free in water, so ask which anion sits outside.

1. Free ions only. In water, a complex splits into the complex ion and the counter-ions written outside the square bracket. Ligands inside the bracket stay bonded to the metal and do not react with test reagents.
2. What each test needs. BaCl₂ gives a white precipitate of BaSO₄ only with free SO₄²⁻. AgNO₃ gives a precipitate of AgCl (white) or AgBr (pale yellow) only with free Cl⁻ or Br⁻.

   Compound	Free anion	With BaCl₂	With AgNO₃
   [Co(NH₃)₅Br]SO₄	SO₄²⁻	white BaSO₄	none (Br is bonded to Co)
   [Co(NH₃)₅SO₄]Br	Br⁻	none (SO₄²⁻ is bonded to Co)	pale yellow AgBr
   [Pt(NH₃)₄Cl₂]Br₂	Br⁻	none	pale yellow AgBr
   [Pt(NH₃)₄Br₂]Cl₂	Cl⁻	none	white AgCl

3. So. Only (a) releases free sulphate and no free halide, so it gives a white precipitate with BaCl₂ but nothing with AgNO₃. The answer is (a).

The idea to keep. In a coordination compound, only the ions outside the square bracket react with test reagents. Ligands inside the bracket are bonded to the metal and stay hidden from BaCl₂ and AgNO₃.

Answer from the NTA final answer key. Solution by PrepWiser.