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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?
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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