Which anion of a sodium salt gives a white precipitate upon heating with a magnesium salt?

Which anion's aqueous solution gives a white precipitate with $AgNO_3$,which is soluble in excess $NH_4OH$?

$[X] +$ dil. $H_{2}SO_{4} \longrightarrow [Y]:$
Colourless, suffocating gas
$[Y] + K_{2}Cr_{2}O_{7} + H_{2}SO_{4} \longrightarrow$
Green colouration of solution
Then, $[X]$ and $[Y]$ are

Chromyl chloride vapours are dissolved in $NaOH$ and acetic acid and lead acetate solution is added,then

The brown ring test is used for the detection of which radical?

$A$ white sodium salt dissolves readily in water to give a solution which is neutral to litmus. When silver nitrate solution is added to the aforementioned solution,a white precipitate is obtained which does not dissolve in dilute nitric acid. The anion is: