The addition of dilute NaOH to Cr³⁺ salt solution will give :

Solution

Chemistry ma’am · 4 steps

Dilute alkali gives a precipitate. Then ask what that green jelly really is: a hydrated oxide.

1. Dilute NaOH. A small amount of OH⁻ added to a Cr³⁺ solution gives a green gelatinous precipitate. Nothing dissolves yet, so the product is a solid, not a soluble complex.

   2Cr³⁺ + 6OH⁻ → Cr₂O₃ + 3H₂O

2. What the solid is. No definite compound with the formula Cr(OH)₃ is isolated. The precipitate is chromium(III) oxide holding a variable amount of water, Cr₂O₃(H₂O)ₙ. Writing it as Cr(OH)₃ is only a convenient shorthand.
3. Excess NaOH. The hydrated oxide is amphoteric. In excess alkali it dissolves to give soluble hydroxo anions such as [Cr(OH)₄]⁻ or [Cr(OH)₆]³⁻. That happens only after the precipitate has formed, and these anions stay in solution.
4. So. Dilute NaOH gives a precipitate of Cr₂O₃(H₂O)ₙ, option (b).

The idea to keep. Many M³⁺ ions with dilute alkali give hydrated oxides of variable water content, not exact hydroxides. Excess alkali then dissolves amphoteric ones into soluble hydroxo anions.

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