pH Calculation / ionization of diprotic and polyprotic acids in pure water

[Mao]

okay, just say the question wants you to consider it. NOW WHAT? what do you have to do to find the H+ conc and all that jazz?

It involves a lot of maths. Several simultaneous equations.

you need the Ka value to figure out the H+ concentration like any acid equilibrium question. And then add the concentration of the first ionization and the second one, and use that value to find the ph.

Not that simple. You will need to set up equations which simultaneously satisfy both equilibrium. University level.


In summary:

1. 2nd/3rd/subsequent ionizations are always weak. Often negligibly weak. Ignore their effects when simply dissolving in water.
2. 2nd/3rd/subsequent protons can react with base. Assume full deprotonation in acid/base reactions.
3. If the question states 'assume full ionization', then assume 2nd/3rd/subsequent ionizations are complete. However, this is rare, unscientific and a generally idiotic thing to assume.
4. I reiterate, unless under explicit instruction, do not assume full ionization.
5. You will not be required to perform calculations to determine the extent of 2nd/3rd/subsequent ionization.