Which of the following does not give a precipitate with $CrO_4^{2-}$?

Assign $A, B, C, D$ from the given type of reaction:
$2KCN + Pb(NO_3)_2 \longrightarrow Pb(CN)_2 \downarrow + 2KNO_3$

$H_2S$ gas is passed through an acidified solution containing $Ag^+$,$Cu^{2+}$,and $Zn^{2+}$ ions. Which of these will form a precipitate?

$A$ metal nitrate reacts with $KI$ to give a black precipitate which on addition of excess of $KI$ converts into an orange-colored solution. The cation of the metal nitrate is

Identify ions $A$ and $B$,respectively,based on the following reaction scheme:

In the fifth group of qualitative analysis,$(NH_4)_2CO_3$ is added to precipitate out the carbonates. We do not add $Na_2CO_3$ because