Ignoring second deprotonations
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Ignoring second deprotonations
For the textbook question 6E.3, it says that we can ignore the second deprotonations only when the approximation is justified for diprotic acids. When are the approximations justified??
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Re: Ignoring second deprotonations
I was also wondering this! It seems as if some problems ignore the first deprotonation, while some ignore the second deprotonation (unless I am misinterpreting them). So if anyone knows the answer, it would be greatly appreciated :)
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Re: Ignoring second deprotonations
The textbook stated to estimate the pH of a polyprotic acid for which all deprotonations are weak by using only the first deprotonation equilibrium and assuming that further deprotonation is insignificant. An exception is sulfuric acid, the only common polyprotic acid that is a strong acid in its first deprotonation.
Another way is to see that if is less than about , then the second deprotonation do not affect the pH significantly and can be ignored.
Another way is to see that if is less than about , then the second deprotonation do not affect the pH significantly and can be ignored.
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Re: Ignoring second deprotonations
I believe you ignore the first deprotonation only when you are dealing with a strong acid. Otherwise, for weak acids, you can ignore the second deprotonation.
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Re: Ignoring second deprotonations
I think you can ignore the 2nd deprotonation when the 2nd deprotonation is less than a factor of 10^3 from the first deprotonation. It says this in 6E.1 if you want to learn more.
for example:
tartaric acid (weak acid), C2H4O2(COOH)2
ka1= 6.0×10^−4
ka2 = 1.5×10^−5
in this case, you will do the 2nd deprotonation
for example:
tartaric acid (weak acid), C2H4O2(COOH)2
ka1= 6.0×10^−4
ka2 = 1.5×10^−5
in this case, you will do the 2nd deprotonation
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Re: Ignoring second deprotonations
The first deprotonation is ignored when dealing with a strong acid, for weak acids, ignore the second deprotonation.
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