29. On the basis of the equation pH=-log [H+], the pH of 10-8 mol dm-3 solution of HCl should be 8. However, it is observed to be less than 7.0. Explain the reason.

Concentration 10-8 mol dm-3 indicates that the solution is very dilute. So, we cannot neglect the contribution of H3O+ ions produced from H2O in the solution. Total [H3O+]=10-8+10-7 M. From this we get the value of pH close to 7 but less than 7 because the solution is acidic.
From calculation, it is found that pH of 10-8 mol dm-3 solution of HCl is equal to 6.96.