Fumaric acid has two conjugate base forms, with pKa1=3.02 and pKa2=4.48, at what pH value αHA-...

Fumaric acid has two conjugate base forms, with pK_a1=3.02 and pK_a2=4.48, at what pH value α_HA- got its largest fractions?

Expert Solution

Assume H2A is the diprotic acid, with constants Ka1 and Ka2 for each dissociation

1st dissociation:

H2A <-> H+ + HA- Ka1

2nd dissociation

HA- <-> H+ + A-2 Ka2

note that HA- is present in both dissociations

if we want HA- to be the most value, then, this is only true in the half point between pKa1 and pKa2

that is:

pH = 1/2*(pKa1 + pKA2)

pH = 1/2*(3.02+4.48) = 3.75

Then, the pH value in which HA- is the highest value can be achieved when pH = 3.75

if pH is less than 3.75, expect: H2A > HA- > A-2

if pH is more than 3.75, expect, H2A < HA- < A-2

then

if pH = HA- then, H2A and A-2 will be min and HA- is max

queen_honey_blossom answered 2 years ago

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