Question

The Ksp of FeS = 4 × 10^–19 at 298 K. The minimum concentration of H⁺ ions required to prevent the precipitation of FeS from a 0.01 M solution Fe²⁺ salt by passing H₂S is –

(given$\frac { \left[ \mathrm { H } ^ { + } \right] ^ { 2 } \left[ \mathrm {~S} ^ { - 2 } \right] } { 0.1 }$ = 1 × 10^–21)

• A. 1.6 × 10^–3 M
• B. 2.5 × 10^–4 M
• C. 2.0 × 10^–2 M
• D. 1.2 × 10^–4 M

Answer 1

Answer: (A)

1.6 × 10^–3 M

Answer 2

[Fe²⁺] [S^–2] = 4 × 10^–19 ̃ [S^–2] = $\frac { 4 \times 10 ^ { - 19 } } { 1 \times 10 ^ { - 2 } }$ = 4×10^–17 M

In order to precipitate FeS, [S^–2] required is 4 × 10^–17 M from 0.01 M Fe²⁺ salts.

Now $\frac { \left[ \mathrm { H } ^ { + } \right] ^ { 2 } \left[ 4 \times 10 ^ { - 17 } \right] } { 0.1 }$ = 1 × 10^–21

̃ [H⁺]² = 2.5 × 10^–6

̃ [H⁺] = 1.6 × 10^–3