Question

The correct option(s) to distinguish nitrate salts of $M{n}^{2+}$ and $C{u}^{2+}$ taken separately is/are:

• A. $M{n}^{2+}$ shows the characteristic green colour in the flame test
• B. Only $C{u}^{2+}$ shows the formation of precipitate by passing $H_{2}S$ in acidic medium
• C. Only $M{n}^{2+}$ shows the formation of precipitate by passing $H_{2}S$ in faintly basic medium
• D. $C{u}^{2+}/Cu$ has a higher reduction potential than $M{n}^{2+}/Mn$ (measured under similar conditions)

Answer 1

Answer: (B), (D)

The correct options are
B Only $C{u}^{2+}$ shows the formation of precipitate by passing $H_{2}S$ in acidic medium
D $C{u}^{2+}/Cu$ has a higher reduction potential than $M{n}^{2+}/Mn$ (measured under similar conditions)

a) Both $M{n}^{2+}$ and $C{u}^{2+}$ show characteristic green colour.

b) Passing $H_{2}S$ in acidic medium precipitates only $C{u}^{2+}$ ions because of its low $K_{sp}$ value. It requires only low concentration of $S^{2-}$ ion whereas the $K_{sp}$ value of $M{n}^{2+}$ ion is high and it requires high concentration of $S^{2-}$ ions which can be possible only in basic medium.
$H_{2}S\rightleftharpoons 2H^{+}+S^{2-}$
$N{H}_{4}OH\rightleftharpoons N{H}_4^{+}+O{H}^{-}$
In the basic medium, the $H^{+}$ ions are consumed by $O{H}^{-}$ ions which shifts the equilibrium in the forward direction and increases the concentration of $S^{2-}$ ions.

d) $C{u}^{2+}/Cu$ has a higher reduction potential than $M{n}^{2+}/Mn$ (measured under similar conditions).