Question

Alex was given a solution of an unknown salt Y for analysis. He performed the following tests and recorded his observations: To a part of the solution Y, he added silver nitrate solution and obtained a white precipitate which was soluble in ammonium hydroxide solution. To the remaining solution he added a few drops of sodium hydroxide solution and obtained a pale blue precipitate.

Based on the observations made by Alex, identify:

1. the anion and
2. the cation present in salt Y.

Answer 1

(a) The anion is the chloride ion (Cl⁻). When silver nitrate (AgNO₃) is added to a chloride solution, it forms a white precipitate of silver chloride (AgCl). This precipitate is specifically known for being soluble in ammonium hydroxide (NH₄OH) because it forms a clear complex called diamminesilver(I) chloride.

\[\ce{NaCl + AgNO3 -> AgCl↓(white ppt) + NaNO3}\]

(b) The cation is the Copper(II) ion (Cu²⁺). The formation of a pale blue precipitate upon the addition of sodium hydroxide (NaOH) is the characteristic test for Copper(II) ions. The precipitate is copper(II) hydroxide [Cu(OH)₂].

\[\ce{CuCl2 + 2NaOH -> Cu(OH)2↓(pale blue ppt) + 2NaCl}\]