Calculating pH of Weak Diprotic Acid

[Barbara Soliman 1G]

Could someone explain how to find the pH of a weak diprotic acid? I'm not really understanding it. If someone could help with 6E.3 in the textbook, that would be great! Thanks!

[Shria G 2D]

Hi,
I think the textbook said that after the first deprotonation, subsequent deprotonations don't affect the pH significantly as long as Ka2 is less than Ka1/1000 which is for most acids except sulfuric acid and a few others. So for 6E.3, I ignored the second deprotonation and just calculated normally as if it only donated 1 proton. For example, for part a, Ka1 = 4.3x10^-7 so I set up an ICE table and equated Ka1 to x^2/(0.010-x) and I solved for x which equals [H3O+] so then I could find the pH. For the exceptions where you have to take into account the second deprotonation, I think it would be similar but you would have to set up the ICE table as if there were already [H3O+] present in the solution from the first deprotonation (like in 6E.1). I hope that helps!

[Cadence Chang]

Hi! To calculate the pH of a weak diprotic acid, you use the first Ka value since the subsequent deprotonations have super small and negligible Ka values. One exception is sulfuric acid; the first protonation is considered strong, so the total concentration of H+ ions would be the concentration of H+ ions from the first strong deprotonation plus the concentration of H+ ions from the second weak deprotonation (you would use the second Ka value to calculate the H+ concentration as if it were a weak acid).

[Sneha Anantharaman2D]

Hi! Basically to calculate the pH of a weak diprotic acid, you use the first Ka value, but if the total concentration of H+ is strong (first protonation), then you need to use the total concentration and use the second Ka value.