Pure water ionises to give hydrogen and hydroxyl ions with the concentrations of H+ and OH– at 10-7 M each.
This makes the pH of pure water 7. Water also ionises in aqueous solutions (i.e. solutions of compounds, e.g. NaCl, where water is the solvent). We call an aqueous solution of pH 7, a neutral solution. If the pH of an aqueous solution is less than 7, [H+] > [OH–] and the solution is termed acidic. If the pH of the aqueous solution is more than 7, [H+] < [OH–] and the solution is basic or alkaline.
Question
20.0 ml of 0.0025 M of HCl is added to 50.0 ml of water. What is the pH of the final solution? Why is it not necessary to consider H+ from the dissociation of water in the computation?
Answer
No. of moles of H+ in 20 ml of HCl =
[H+] in 70 ml of solution =
We do not consider the concentration of H+ from water as it is insignificant. According to Le Chateliers’ principle, the presence of H+ from aqueous HCl causes the equilibrium of the dissociation of water to shift to the left, reducing the concentration of H+ in water to less than 10-7 M. Even if we assume that , the resulting pH value of is still 3.15.
