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

At $27^{\circ} C$, $100 \ mL$ of $0.5 \ M$ $HCl$ is mixed with $100 \ mL$ of $0.4 \ M$ $NaOH$ solution. To this resultant solution, $800 \ mL$ of distilled water is added. What is the $pH$ of the final solution?

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 $10^{-8} \ M \ HCl$ solution is

The $pH$ of $0.0001 \ N$ solution of $KOH$ will be

When $50 \, mL$ of $0.4 \, N$ $HCl$ and $50 \, mL$ of $0.2 \, N$ $NaOH$ solutions are mixed,the $pH$ of the resulting solution will be ........