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Mariam_6809
#1 Posted : Monday, June 21, 2021 12:58:37 AM
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For question 103 from the Question Pack, is there a more definitive way of knowing the pH for an equivalence point? For example, what if we were given 2 pHs above 7?

And what would the equivalence point be for a weak acid and a weak base?
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INSTR_Katrina_128
#2 Posted : Tuesday, June 22, 2021 5:44:14 PM
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Great question, Mariam.

I think to think of the equivalence point as a tug-of-war. Whoever is STRONGER will win the tug-of-war, and the pH of the equivalence point will be more towards that particular species' pH.

For example, if you have a strong base and a weak acid, then the equivalence point will be > 7 (because the strong base will win). That's what we have in this case!

As for what the equivalence point is EXACTLY, that requires some more exact calculations. The MCAT will not do that to you. They may give you a titration curve and get you to figure it out from there, though. So make sure you know how to interpret the titration curve! (Hint: it's the vertical part)

If you have a weak acid and a weak base, one of them will still be stronger than the other. You could be given that information in the form of pKas.

For example: the pKa of dichloroacetic acid is 1.2 and the pKa of trimethylamine is 9.8. Since dichloroacetic acid is FURTHER AWAY from neutral (7) than trimethylamine in terms of pKa, IT IS THE STRONGER of the two. Therefore, we can expect that the equivalence point will be < 7. Hehehehe. 😎
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