Question

For eg: If we are calculating pH of the soln of a strong acid, we shall do this: pH= - log [H+]

Where H+ is the concentration coming from that acid.
Why shall we not add to this concentration, the H+ ions coming from water itself (By the autoionization of water) before calculating the pH?
Does water not ionize in presence of the acid/base?

Answer 1

The concentration of H+ ions from water autoionization is negligible compared to that from a strong acid when calculating pH, which is why it is typically ignored unless dealing with very dilute or weak acid solutions.

Step-by-step explanation:

When calculating the pH of a solution containing a strong acid, we generally consider only the concentration of hydrogen ions (H+) coming from the acid. Although water does undergo autoionization producing both H+ and OH- ions, the concentration of H+ produced by this process (1 × 10-7 moles per liter at 25°C) is negligible compared to the concentration of H+ from a strong acid.

Therefore, in most cases, it is not necessary to account for the H+ ions from the autoionization of water for strong acid solutions. It's only in the case of very dilute solutions or when dealing with weak acids where such contributions from water may need to be considered.