HELP WITH THIS Q Polyprotic Acids: Citric Acid Species at Various pHs Citric acid

Citric acid, a tricarboxylic acid important in intermediary metabolism, can be symbolized as HA. Its dissociation reactions are

H2A <-> H+ +H2A- pk1 = 3.13
H2A <-> H+ +HA2- pk2=4.76
HA2- <->H+ + A3- pk3=6.40

If the total concentration of the acid and its anion forms is 0.02M, what are the individual concentrations ofH3A, H2A-, HA2- and A3- at pH 5.2?

Have worked out using H-H equation and tried to equal concentrations to 0.02 then unsure what to do.

Reply 1

TypicalNerd

Original post by D0ddi3

Recall that Ka = [H3O^+][A^-] / [HA]

You could say [A^-] / [HA] = Ka / [H3O^+]

From this, could you work out the ratio of H3A to H2A^- to HA^2- to A^3-? How might that help?

(edited 1 month ago)

Reply 2

TypicalNerd

Original post by TypicalNerd

Recall that Ka = [H3O^+][A^-] / [HA]

You could say [A^-] / [HA] = Ka / [H3O^+]

From this, could you work out the ratio of H3A to H2A^- to HA^2- to A^3-? How might that help?

Alternatively, you can consider a similar strategy where you first multiply the successive Ka expressions together:

e.g Ka1 x Ka2 = [H3O^+][H2A^-] / [H3A] x [H3O^+][HA^2-] / [H2A^-]

= [H3O^+]^2 [HA^2-] / [H3A]

So Ka1 x Ka2 / [H3O^+]^2 = [HA^2-] / [H3A]

(And if you do the algebra or spot a pattern, you should get Ka1 x Ka2 x Ka3 / [H3O^+]^3 = [A^3-] / [H3A])

This may actually be the easier way

(edited 1 month ago)