Question

Three test-tubes $A,B,C$ contain $P{b}^{2+},H{g}_2^{2+}$ and $A{g}^{+}$ (but unknown). To each aqueous solution $NaOH$ is added in excess. Following changes occur in following tes-tubes :
A : black precipitate
B : brown precipitate
C : white ppt. but dissolves in excess of $NaOH$.

$A,B$ and $C$ respectively contain:

• A. $P{b}^{2+},H{g}_2^{2+}$ and $A{g}^{+}$
• B. $H{g}_2^{2+},A{g}^{+},P{b}^{2+}$
• C. $A{g}^{+},P{b}^{2+},H{g}_2^{2+}$
• D. $A{g}^{+},H{g}_2^{2+}$ and $P{b}^{2+}$

Answer 1

The correct option is B $H{g}_2^{2+},A{g}^{+},P{b}^{2+}$

Black finely divided mercury metal and yellow mercury(II) oxide $\left(HgO\right)$ are precipitated by $NaOH$:

$H{g}_2^{2+}\left(aq\right)+2O{H}^{-}\left(aq\right)\to Hg\left(l\right)$ (black) $+HgO$ (yellow) $+H_{2}O\left(l\right)$

$A{g}^{+}+NaOH\to AgOH+N{a}^{+}$

$2AgOH\to A{g}_{2}O$ (Brown ppt) $+H_{2}O$

$P{b}^{2+}+NaOH\to Pb(OH{)}_2$

$Pb(OH{)}_2\to PbO$ (white ppt) $+H_{2}O$

$PbO+$ excess $NaOH\to$ does not dissolve.