Autoprotolysis

[Renee Grange 1I]

When do we need to take into account the autoprotolysis of water needs to be taken into account?

[Hannah Lee 2F]

The autoprotolysis of water can be used to link the concentrations of H3O+ and OH- in an aqueous solution via the autoprotolysis constant, Kw. Because Kw is essentially an equilibrium constant, if [H3O+] increases due to acid being added into a solution, [OH-] must decrease in order to keep Kw the same, and vice versa, producing the equation: [H3O+][OH-] = Kw = 10^-14. (You can exclude H2O from the constant because it is basically a pure liquid, and its concentration does not change in any measurable way.

In a similar fashion, Kw links pOH and pH in an aqueous solution in the equation pH + pOH = 14 (which you can find by taking the negative log of the previous equation).

[Sukanya Mohapatra 2G]

If the concentration from the addition of an acid is very small, then it will not affect the pH that much. So autoprotolysis will be the determining factor in the pH.