On the basis of the equation `ph=-log[H^(+)]`,the ph of `10^(-8)` mol `dm^(-3)` solution of HCI should be 8. However, it is observed to be less than 7.0. Explain the reason.

Explanation

pH of `10^(-8)` M HCl solution is not 8 because this concentration is so low that `H^(+)` ion produced from `H_(2)O` in the solution (viz. `10^(-7)M`) connot be neglected. Total `[H^(+)] = 10^(-8) + 10^(-7)`M. Solving and calculating pH, we get the value close to 7 but les than 7 as the solution is acidic