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Does the amount of substance of acid always have to equal the amount of substance of base at the equivalence point? I know it "works" for monoprotic acids and monobasic bases, but what if we had a diprotic acid and monobasic base?

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  • $\begingroup$ It sometimes depends on the indicator you are using for the titration. Phenolphthalein gives its equivalence point at a pH of 10-12, which is beyond the pH of 7. $\endgroup$ – Pritt Balagopal Apr 19 '17 at 2:02
  • $\begingroup$ Please use the correct term amount of substance for number of moles, which would be saying number of kilograms when referring to the mass. $\endgroup$ – Martin - マーチン Apr 19 '17 at 4:53
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We speak in this case of multiple equivalence points.

As a concrete example, if we are titrating $\ce{H2SO4}$ with $\ce{NaOH}$, then the first equivalence point occurs when all the $\ce{H2SO4}$ has reacted to form $\ce{HSO4-}$, and the second equivalence point when all the $\ce{HSO4-}$ has further reacted to form $\ce{SO4^2-}$. If you started with 1 mole of acid in solution, then adding 1 mole of base gets you to the first equivalence point, and another mole (2 moles total) gets you to the second equivalence point. In the same vein, one might consider triprotic acids, which have three equivalence points, and so on.

Incidentally, an interesting case to think about might be that of a titration between a diprotic acid and a dibasic base.

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  • $\begingroup$ So what you're saying is that they require the same number of moles to reach the 1st, then 2nd, etc. equivalence points (if it applies) but in total it would require more moles (two times more) of NaOH to titrate H2SO4 $\endgroup$ – ChemStudent Apr 19 '17 at 2:17
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    $\begingroup$ @ChemStudent, yes, that's correct. $\endgroup$ – a-cyclohexane-molecule Apr 19 '17 at 2:18
  • $\begingroup$ Oh ok that makes a lot more sense now. Thank you for responding! Have an awesome day!!! $\endgroup$ – ChemStudent Apr 19 '17 at 2:29

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