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Problem 6E.3 from the textbook says, "Calculate the pH of each of the following solutions of diprotic acids at 25 °C, ignoring second deprotonations only when the approximation is justified." For each part of the question, the solutions manual says, "Because Ka2 << Ka1, the second ionization can be ignored." What is the cutoff for when the second ionization can be ignored? Thank you!
Just for clarification, the reason why the cut off is Ka < 10^-4 (rather than the usual < 10^-3) is because Dr. Lavelle wanted us to be extra careful when using 10^-3 as a cut off because something could be a very large number multiplied with 10^-3, which would make it closer to 10^-2 than 10^-4 :)! You can also check your approximations using the percent ionization check and make sure it's less than 5% :)!
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