On the basis of this 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.

On the basis of this equation pH = -log [H^+], the pH of 10^(-8) "mol

1.	On the basis of this 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.
Answer» Solution :The CONCENTRATION `10^(-3)` mol `"dm"^(-3)` INDICATES that the solution is very dilute. So, we cannot neglect the contribution of `H_3O^+` ions PRODUCED from `H_2O` in the solution. Total `[H_3O^+]` = `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.