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

What is the $pH$ of $10^{-9} \ M$ aqueous $HCl$ solution?

Calculate the $pH$ of the following solutions:
$(a)$ $0.1 \ M \ HCl$
$(b)$ $0.1 \ M \ H_2SO_4$
$(c)$ $0.1 \ M \ HNO_3$
$(d)$ $0.1 \ M \ NaOH$
$(e)$ $0.1 \ M \ KOH$
$(f)$ $0.1 \ M \ Ba(OH)_2$

Calculate the $pH$ of $0.01 \ M$ sulphuric acid.

$0.837 \ g$ of $Ba(OH)_2$ is dissolved in $100 \ mL$ of solution. Calculate the $pH$ of the solution. (Atomic masses: $Ba = 137, O = 16, H = 1$)

If $100 \, mL$ of a solution with $pH = 3$ is mixed with $400 \, mL$ of another solution with $pH = 3$,what will be the $pH$ of the resulting mixture?