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when to ignore the second ionization constant

Posted: Tue Jan 22, 2019 12:08 am
by Josephine Lu 4L
In 12.81, the answer says that since Ka2 << Ka1, the second ionization can be ignored. What is the criteria to knowing when the second ionization is very small?

Re: when to ignore the second ionization constant

Posted: Tue Jan 22, 2019 8:55 am
by EllerySchlingmann1E
Not sure if this is a foolproof method, but with those questions, as long as Ka2 is less than Ka1, I think you can ignore it. I did not see any specific parameters either.

Re: when to ignore the second ionization constant

Posted: Tue Jan 22, 2019 10:57 am
by Kirsty Star 2H
On one problem regarding H2SO4, I was confused about the same thing, since the textbook just said Ka1 was "strong" and didn't actually have a value. In this case I used Ka2 which was given, however I think that if Ka2 is significantly less than Ka1, then you're just supposed to use Ka1

Re: when to ignore the second ionization constant

Posted: Tue Jan 22, 2019 1:55 pm
by klarratt2
This is what the textbook says (7th edition, page 484):

"Except for sulfuric acid (and a few other rare cases), to calculate the pH of a polyprotic acid, use Ka1 and take only the first deprotonation into account; that is, treat the acid as a monoprotic weak acid. Subsequent deprotonations do take place, but provided Ka2 is less than about Ka1/1000, they do no affect the pH significantly and can be ignored."

So I think is Ka2 is less than 0.1% of Ka1, you can ignore it.