$pH$ scale was introduced by

$pH + pOH$ is equal to

The hydrogen ion concentration of a solution is $3.98 \times 10^{-6} \text{ mole per liter}$. The $pH$ value of this solution will be

The $pH$ of a solution is $5.0$. To this solution,sufficient acid is added to decrease the $pH$ to $2.0$. The increase in hydrogen ion concentration is $...$ times.

Calculate $[OH^{-}]$ and $pH$ of a solution containing $[H^{+}] = 0.001 \ M$.

On adding which of the following,the $pH$ of $20 \ mL$ of $0.1 \ N \ HCl$ will not alter?