Concentration of $H^{+}$ ions in $10^{-4} \ M \ HCl$ solution is

The concentration of an aqueous $HCl$ solution is $10^{-9} \ N$. What is the $pH$ of the solution?

What is the $pH$ of a solution in which $25.0 \ mL$ of $0.1 \ M \ NaOH$ is added to $25 \ mL$ of $0.08 \ M \ HCl$ and the final solution is diluted to $500 \ mL$?

Find the number of equivalents of $HBr$ present in $100 \ mL$ of its solution whose $pH$ is $4$.

How many liters of water must be added to $1 \ L$ of an aqueous $HCl$ solution with $pH = 1$ to make the $pH$ of the resulting solution equal to $2$ (in $L$)?

$75 \ mL$ 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?