Which of the following radicals does not liberate gas with ($Zn$ + dil. $HCl$) on warming?

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

$A$ white precipitate was formed when $BaCl_{2}$ was added to the water extract of an inorganic salt. Further,a gas $X$ with a characteristic odour was released when the formed white precipitate was dissolved in dilute $HCl$. The anion present in the inorganic salt is ..... .

For the identification of which ion is $Na_2CO_3$ $NOT$ useful?

$A$ sodium salt on treatment with $MgCl_2$ gives white precipitate only on heating. The anion of the sodium salt is

$A$ salt gives violet vapours when treated with concentrated $H_2SO_4$,it contains