At 298 K, p^H of 10^-8 M HCl solution is nearly

[H⁺] of HCl is 10^-8 M.

This is less than [H⁺] of H₂O ( 10^-7 M ).

Thus [H⁺] of H₂O cannot be neglected.

Overall [H⁺] = [H⁺]_HCl + [H⁺]_water

Overall [H⁺] = 10^-8 M + 10^-7 M

Overall [H⁺] = 1.1 $\times$10^-7 M

               p^H = 6.956 ;