How much water do you need to add to 10 mL of a solution of HCl with a pH of 4 to change the pH to 5?
Since the pH is 4 the concentration of H+ in the solution is 10^-4. Therefore no.of moles of H+ from hcl ( neglecting autoprotolysis of h2o) is 10^-3 millimoles which remains constant as hcl dissociates completely even when we add more h2o. To get pH=5, we must make conc. to 10^-5 thus we need this solution to have a volume of 100ml( final volume =number of moles/concentration needed), thus we add 90ml h2o.