What will be the $OH^{-}$ concentration in an aqueous solution having $pH = 8$?

$A$ 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 $Z$.

In an acidic aqueous solution,the value of $[H_3O^+]$ is $.......$.

Which of the following gives the correct increasing order of acidic strength?

Which of the following is an oxide of metal?

$(i)$ Name the products obtained in the chlor-alkali process.
$(ii)$ Name the gases liberated at the anode and at the cathode,respectively.
$(iii)$ Name the solution which is used as raw material in this process.