What is the $pH$ of a solution containing $5 \times 10^{-4} \text{ M } H^+$ ions?

In a solution of $pH = 5$,more acid is added in order to reduce the $pH$ to $2$. The increase in hydrogen ion concentration is $......$ times.

The $pH$ of a $10 \ mL$ aqueous solution of $HCl$ is $4$. The amount of water to be added to this solution in order to change its $pH$ from $4$ to $5$ is $..... \ mL$

The $pH$ of a $H_2SO_4$ solution is $5.0$. If more $H_2SO_4$ is added to this solution,the $pH$ becomes $2.0$. The concentration of $H^{+}$ ions will become how many times the initial concentration?

What is the $pH$ of a $2.6 \times 10^{-8} \ M \ H^{+}$ ion solution?
$(\log 2.6 = 0.4150)$

$40 \ mg$ of pure sodium hydroxide is dissolved in $10 \ L$ of distilled water. The $p^H$ of the solution is ..........