Homework 6E.3

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Jessica Booth 2F
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Joined: Fri Aug 30, 2019 12:18 am

Homework 6E.3

Postby Jessica Booth 2F » Sun Jan 19, 2020 1:44 pm

The question is: Calculate the pH of each of the following solutions of diprotic acids at 25 ignoring second deprotonations only when the approximation is justified. How do you tell if ignoring the second deprotonation is justified?

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Re: Homework 6E.3

Postby SnehinRajkumar1L » Sun Jan 19, 2020 1:56 pm

I believe if the Ka for the second dissociation is small enough, you can ignore the second one because little to no acid will deprotonate.

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Re: Homework 6E.3

Postby JChen_2I » Sun Jan 19, 2020 2:22 pm

I think if the second Ka value is less than 10^-3 then the second protonation can be ignored

Alex Chen 2L
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Re: Homework 6E.3

Postby Alex Chen 2L » Sun Jan 19, 2020 2:34 pm

According to the textbook, most polyprotic acids are weak acids even from the first deprotonation, so the second and following deprotonations have little effect on the pH of the solution. In numbers, if Ka2 is less than 1/1000th of Ka1, then the acid can be essentially be treated as a monoprotic acid in terms of pH calculation. The only common polyprotic acid where you would actually add the H30+ concentrations for both dissociations would be sulfuric acid since it is a strong acid in its first deprotonation.

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