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

[burbs]

What? I thought that second ionisation is a negligible change to pH for our calculations or something?

[Bismuth]

A well written question would just have "(assume complete ionization)" at the end like my exam did. But, I'll vouch that for these questions (in unit 2 anyway), you're just better off assuming complete ionization. I discussed this with my teacher a few months ago and that's what he said. Either way, just give some justification for your answer at the end.

[Panicmode]

What? I thought that second ionisation is a negligible change to pH for our calculations or something?

That's exactly what I just said x_x.

[Mao]

It is true that second ionization and any subsequent ionizations are all weak. E.g. 0.01M sulfuric acid would have a H+ concentration slightly higher than 0.01M, but nowhere near 0.02M.

For Unit 4 though, you do not learn how to solve the concentrations of polyprotic acids properly. That you learn at uni. You just need to appreciate the fact that second/third ionization are weak.

For unit 2, you need to appreciate the fact that a polyprotic acid can donate up to twice or three times the amount of H+ than a monoprotic acid.

[Graphite]

What? I thought that second ionisation is a negligible change to pH for our calculations or something?

Its not always negligible, its negligible normally in water.

[burbs]

So would, as someone said, blah blah blah blah blah"assuming complete ionization "be sufficient? Oooooooor have i got it wrong?

[Graphite]

Yes, just say assuming complete ionisation: blah blah

[burbs]

Thanks mate.

[zool3]

so to conclude, we ignore the second ionisation! wooot you guys had a good discussion

[Panicmode]

However, if the question say Assume complete ionisation DO NOT IGNORE IONISATION OF THE SECOND PROTON!!!!!

[burbs]

So would, as someone said, blah blah blah blah blah"assuming complete ionization "be sufficient? Oooooooor have i got it wrong?

Yes, just say assuming complete ionisation: blah blah

However, if the question say Assume complete ionisation DO NOT IGNORE IONISATION OF THE SECOND PROTON!!!!!

now I confus

[Panicmode]

Ok, for all intensive purposes, ignore the second proton and state "Ionisation of the second proton only happens to a very limited extent and so is negligble in the following calculations."

UNLESS the question states, "Assume full ionisation" in which case you assume both ionised completely.

[taqi]

Well in our SAC, we had to calculate the pH using the second ionization as well. Basically i think, if they provide you with a Ka value of the second or third ionization - Use it. If not, then assume its negligible. But don't assume that both reactions are complete, only the first one.

[Zebra]

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?

[taqi]

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.