In the test of sulphate radical,the white precipitate of sulphate is soluble in

$M \text{ (Salt)} + \text{Dil. } HCl \xrightarrow{\text{Warm}} N \uparrow + P \downarrow$. Gas $N$ changes the colour of $FeSO_4$ solution into a yellow solution. Then,salt $M$ in the above reaction is:

$A$ salt is heated first with dil. $H_2SO_4$ and then with conc. $H_2SO_4$. No reaction takes place. It may be

$A$ mixture is heated with dilute $H_2SO_4$ and the lead acetate paper turns black due to the evolved gases. The mixture contains:

Which of the following will not produce a precipitate with $AgNO_3$ solution?

$X + H_2SO_4 \to Y$ (a colorless gas with a pungent smell).
$Y + K_2Cr_2O_7 + H_2SO_4 \to$ green colored solution.
What are $X$ and $Y$?