To an aqueous solution containing ions such as $\mathrm{Al}^{3+}, \mathrm{Zn}^{2+}, \mathrm{Ca}^{2+}, \mathrm{Fe}^{3+}, \mathrm{Ni}^{2+}, \mathrm{Ba}^{2+}$ and $\mathrm{Cu}^{2+}$ was added conc. HCl , followed by $\mathrm{H}_{2} \mathrm{~S}$. The total number of cations precipitated during this reaction is / are:

JEE Main · 2021 · 27 Jul 2021 · Shift 2 · Practical and Analytical Chemistry

To an aqueous solution containing ions such as

Al^{3 +}, Zn^{2 +}, Ca^{2 +}, Fe^{3 +}, Ni^{2 +}, Ba^{2 +}

and

Cu^{2 +}

was added conc. HCl , followed by

H_{2} S

.
The total number of cations precipitated during this reaction is / are:

(A)4
(B)1✓ Correct
(C)3
(D)2

Answer: (B)

Solution

Concentrated hydrochloric acid suppresses the ionisation of

H_{2} S

through the common ion effect, so the sulphide ion concentration stays very low and only sulphides with the smallest solubility products can separate. Of the ions present, only

C u^{2 +}

belongs to this acid-insoluble group and comes down as black CuS.

Z n^{2 +}

and

N i^{2 +}

need an ammoniacal medium before their sulphides appear,

A l^{3 +}

and

F e^{3 +}

are brought down only as hydroxides, and

C a^{2 +}

and

B a^{2 +}

belong to still later groups. The number of cations precipitated is one.

JEE Main · 2021 · 27 Jul 2021 · Shift 2 · Single correct option

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JEE Main · 2021 · 27 Jul 2021 · Shift 2 · Practical and Analytical Chemistry

Solution

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