Which ions are responsible for the chemical properties of bases in their aqueous solution?

$A$ metal carbonate $X$ on reacting with an acid gives a gas which when passed through a solution $Y$ gives the carbonate back. On the other hand,a gas $G$ that is obtained at anode during electrolysis of brine is passed on dry $Y$,it gives a compound $Z$,used for disinfecting drinking water. Identify $X$,$Y$,$G$ and $Z$.

What is the unit of molar mass?

What will be the $pH$ of an aqueous solution of $NH_{4}Cl$?

Crystals of a substance changed their colour on heating in a closed test tube but regained it after sometime when they were allowed to cool down. Name the substance and write its formula and explain the phenomenon involved.

Which of the following are present in a dilute aqueous solution of hydrochloric acid?