On adding dilute ammonia solution to a colourless solution of a salt, a white gelatinous precipitate appears. This precipitate however dissolves on addition of excess ammonia solution. Identify (choose from $N a, A l, Z n, P b, F e$ )

(a) Which metal salt solution was used?

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Solution

Addition of ammonia solution to $Z n S O_{4}$ :

Dropwise :

When $N H_{4} O H$ reacts with $Z n S O_{4}$ a gelatinous white precipitate $(Z n (O H)_{2})$ is formed.

$Z n S O_{4} + 2 N H_{4} O H \to Z n (O H)_{2} ↓ + (N H_{4})_{2} S O_{4}$
(colourless) (white ppt) (colourless)

In excess :

Due to the formation of a soluble complex (Tetrammine zinc(II) hydroxide) with the excess of $N H_{4} O H$ , white ppt dissolves.

$Z n (O H)_{2} + 4 N H_{4} O H \to [Z n (N H_{3})_{4}] (O H)_{2} + 4 H_{2} O$
(excess) (colourless solution)

Hence, a solution of Zinc salt was used.

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