A solution contains 0.010 M each of Cu^2+, Cd^2+, Zn^2+ and Ni^2+ and is saturated with H2S (0.10 M) at [H^+] = 0.30 M. Use K_a1K_a2 = 1.1x10^-21 for H2S and K_sp: CuS 6x10^-36, CdS 8x10^-27, ZnS 3x10^-22, NiS 3x10^-21. Which sulphides precipitate?

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc100

A solution contains $0.010 M$ each of $Cu X^{2 +}$ , $Cd X^{2 +}$ , $Zn X^{2 +}$ and $Ni X^{2 +}$ and is saturated with $H X_{2} S$ ( $0.10 M$ ) at $[H X^{+}] = 0.30 M$ . Use $K_{a 1} K_{a 2} = 1.1 \times 1 0^{- 21}$ for $H X_{2} S$ and $K_{s p}$ : $CuS$ $6 \times 1 0^{- 36}$ , $CdS$ $8 \times 1 0^{- 27}$ , $ZnS$ $3 \times 1 0^{- 22}$ , $NiS$ $3 \times 1 0^{- 21}$ . Which sulphides precipitate?

[S X^{2 -}] = K_{a 1} K_{a 2} [H X_{2} S] / [H X^{+}]^{2} = 1.1 \times 1 0^{- 21} (0.10) / (0.30)^{2} = 1.2 \times 1 0^{- 21} M

. Ionic product

= 0.010 \times 1.2 \times 1 0^{- 21} = 1.2 \times 1 0^{- 23}

. It exceeds

K_{s p}

of CuS and CdS but not of ZnS or NiS, so Cu and Cd (group II) precipitate while Zn and Ni stay (group IV).