Find the compound $A$ from the following reaction sequences:
$A$ $\xrightarrow{\text{aqua-regia}} B$ $\xrightarrow[(2) CH_3COOH]{(1) KNO_2 \mid NH_4OH} \text{yellow ppt}$

Which of the following solutions will yield a black precipitate when $H_2S$ gas is passed through it?

Choose the correct tests with respective observations.
$(A)$ $CuSO_4$ (acidified with acetic acid) $+$ $K_4[Fe(CN)_6]$ $\rightarrow$ Chocolate brown precipitate.
$(B)$ $FeCl_3 + K_4[Fe(CN)_6]$ $\rightarrow$ Prussian blue precipitate.
$(C)$ $ZnCl_2 + K_4[Fe(CN)_6]$,neutralised with $NH_4OH$ $\rightarrow$ White or bluish white precipitate.
$(D)$ $MgCl_2 + K_4[Fe(CN)_6]$ $\rightarrow$ Blue precipitate.
$(E)$ $BaCl_2 + K_4[Fe(CN)_6]$,neutralised with $NaOH$ $\rightarrow$ White precipitate.
Choose the correct answer from the options given below:

On saturating an aqueous solution of $Cu(II)$,$Pb(II)$,and $Zn(II)$ ions with $H_2S$,which of the following will precipitate?

When $H_2S$ is passed through a mixture containing $Cu^{2+}$,$Ni^{2+}$,and $Zn^{2+}$ in an acidic solution,which ion will precipitate?

$A$ metal salt gives a yellow precipitate with potassium chromate in acetic acid and a white precipitate with dilute sulfuric acid,but it does not give a precipitate with sodium chloride or sodium iodide. What will be the composition of the white precipitate formed when sodium carbonate is added to the solution of this metal salt?