Acid-Base low concentration of weak acid

[alingy1]

My textbook assumes that whenever K-constant of a weak acid is bigger than K-constant of water, the water should be a negligible source of H+ ions.

However, how do we know that the concentration of weak acid could be too low and the H+ ions from water can actually have a significant effect?

 

[Borek]

You are right, and your textbook "overassumes". In most typical cases concentrations of acids are large enough for the assumption to be correct, but in the general case it has to be taken into account.

See the equation 6.9 here: http://www.chembuddy.com/?left=pH-calculation&right=pH-acid-base-solution