At $25^{\circ} \mathrm{C}, K_a$ of formic acid is $1.8 \times 10^{-4}$. What is the $K_b$ of $\mathrm{HCOO}^{-}$?

At $T(\mathrm{~K})$, the following gaseous equilibrium is established.

$$ W+X \rightleftharpoons Y+Z $$

The initial concentration of $W$ is two times to the initial concentration of $X$. The system is heated to $T(\mathrm{~K})$ to establish the equilibrium. At equilibrium the concentration of $Y$ is four times to the concentration of $X$. What is the value of $K_C$ ?

4 mL of ' $X$ volume' $\mathrm{H}_2 \mathrm{O}_2$ on heating gives 80 mL of oxygen at STP. The value of $X$ is

Compound ' $X$ ' is prepared commercially by the electrolysis of brine solution.

Which of the following is not the use of ' $X$ '?