Question

How is second equivalence point calculated?

If 20.20mL of 0.122M NaOH are required to reach the first equivalent point of a solution of citric acid(tripotic acid H3C6H5O7). How many mL of NaOH in total are required to reach the second equivalence point?

Answer 1

The volume needed for each equivalence point is equal. The only difference between each equivalence point is what the height of the steep rise is.

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Here is a real titration curve for maleic acid (a diprotic acid) from one of my students:

(The first steep rise is shorter because the first proton comes off more easily. In fact, `"pK"_(a1) = 1.83` and `"pK"_(a2) = 6.07`, so the first proton is stronger / comes off more easily. In this case the student got `"pK"_(a1) = 2.37` and `"pK"_(a2) = 6.37`.)

And you can see that the volume of strong base needed to get to the first equivalence point and the volume needed to go from there to the second equivalence point are nearly equal, i.e.

`V_(eq1) ~~ "8 mL"` (actual value on the graph was `"8.00 mL"`.)

`V_(eq2) ~~ 2V_(eq1) ~~ "16 mL"` (actual value on the graph was `"16.21 mL"`.)

For you we just have `"20.20 mL"` for the first equivalence point, so `V_(eq2) ~~ bb"40.40 mL"` is required to go from zero to the second equivalence point.