Answer: pH = 9.40
Explanation: pH = - log[H3O^+] .
HCl reacts with water : HCl + H2O -> Cl^- + H3O^+
So [H3O^+] = c(HCl) = 4.0· 10^-10 M
pH = - log(4.0· 10^-10 M) = 9.398
