Which salt produces a white precipitate with $AgNO_3$ solution,which becomes black on standing?

$A$ 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:

Which of the following ions will not give a precipitate with $AgNO_3$?

The salt which responds to dilute and concentrated $H_2SO_4$ is

$A$ mixture when heated with conc. $H_2SO_4$ and $MnO_2$ produces brown fumes due to the presence of:

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