Which of the following has a $pH$ of $12$?

The $pH$ of $1 \ L$ of an aqueous solution of $HCl$ is $1$. How many liters of water must be added to make the $pH$ of this solution $2$ (in $L$)?

The $pH$ of a solution obtained by mixing $50 \ mL$ of $0.4 \ N \ HCl$ and $50 \ mL$ of $0.2 \ N \ NaOH$ is

$75 \ mL$ of $0.2 \ M \ HCl$ is mixed with $25 \ mL$ of $1 \ M \ HCl$. To this solution, $300 \ mL$ of distilled water is added. What is the $pH$ of the resultant solution?

The $pH$ of a $0.001 \, M$ solution of $HCl$ is:

Equal volumes of two solutions of $pH = 3$ and $pH = 5$ are mixed. The resultant $pH$ is