If we do not consider [H3O+] from the ionisation of H2O
then [H3O+] = [HCl] = 10−7M
i.e., pH = 7, which is a pH of a neutral solution. We know that HCl solution is acidic whatever may be the concentration of HCl i.e, the pH value should be less than 7. In this case the concentration of the acid is very low (10−7M) Hence, the H3O+(107M) formeddue to the auto ionisation of water cannot be neglected.
so, in this case we should consider [H3O+] from ionisation of H2O
[H3O+] = 10−7 (from water)
= 10−7(1 + 1)
= 2 × 10−7
pH = −log10[H3O+]
= −log10(2 × 10−7)
= −[log2 + log10−7]
= log2 − (−7).log1010
= 7 − log2
= 7 − 0.3010
= 6.6990
= 6.70
