Consider the following information about the dipodic acid ascorbic acid (H_2As f

Question

Consider the following information about the dipodic acid ascorbic acid (H_2As for short, molar mass = 176.1). The titration curve for 0.12 M disodium acerbate, Na_2As, with standard 0.12 M HC1 is shown schematically below: (a) What major species is(are) present at point III? (b) What is/are the principal ascorbic species present at point IV? Determine the pH at the following points, collecting the answers here - you may use next page for working: (c) At Point III (V_1 + V_2/2 HC1 added) (d) At Point IV (e) Before any HCI was added - estimate with e.g. "> y" or between 2 limits if you cannot calculate it exactly

Explanation / Answer

Now for part before HCl is added. I can do that:

the general reaction, along with the ICE is the following:
r: As2- + H2O -----------> HAs- + OH- Kb = 1x10-14/1.6x10-12 = 6.25x10-3
i: 0.12 0 0
e: 0.12-x x x

6.25x10-3 = x2 / 0.12-x --> Kb is small, and x will be small too, so we can approximate 0.12-x to simply 0.12
6.25x10-3 * 0.12 = x2
x = [OH-] = 0.0274 M
pOH = -log(0.0274) = 1.56
pH = 14-1.56 = 12.44

At point IV we are at the second equivalence point. and in this point we use two times the volume to reach the first equivalence. So if the first equivalence point is calculated as:

pH = pKa1+pKa2/2

Then at the second equivalence point, where we use the double of V, it would be:
pH = pKa1+pKa2 / 4
pH = 4.10 + 4.79 / 4 = 2.225

At point III we are at the 2nd half equivalence point, so pH = pKa1 = 4.1.

At this point, the principal species are HAs- and H2S is forming.

At point IV almost the end point we have H2S

Hope this helps

Hope this helps

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