Diprotic Acids (6E.3)

[Megan Verschuur 1C]

In textbook problem 6E.3 we are told to "ignore the second deprotonations only when the approximation is justified," is this when the Ka is so small you can ignore the (-x) or am I thinking of something else?

[Lea Baskin Monk 1F]

Ignoring the second deprotonation is different than ignoring the (-x) but it uses the same concept. The idea is that because the K value for the second deprotonation is so small, the change in concentration of the compound being deprotonated is negligible and can be ignored (so you only have to calculate the first deprotonation and not the second).

[Jenny Lee 2L]

I also want to clarify that this is because if Ka1 is large, most of the deprotonation happens in the first ionization of the proton. Then if Ka2 is very small, this means that only a very small amount of deprotonation happens, therefore Ka2 barely makes a difference on the pH of the solution— and thus why we can ignore Ka2 and only calculate Ka1.