A diprotic acid, H 2 A, has acid dissociation constants of K a1 = 2.27 * 10 - 4 and K a2 = 4.00 * 10 - 11. Calculate the pH and molar concentrations of H 2 A, HA -, and A 2 - at equilibrium for each of the solutions. A 0.103 M solution of H 2 A.

Question

Celia Morales

Chemistry

13 November, 01:46

A diprotic acid, H 2 A, has acid dissociation constants of K a1 = 2.27 * 10 - 4 and K a2 = 4.00 * 10 - 11. Calculate the pH and molar concentrations of H 2 A, HA -, and A 2 - at equilibrium for each of the solutions. A 0.103 M solution of H 2 A.

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Francis · Answer 1

What is meant by diprotic acid?

A diprotic acid is the type of acid that can undergo one or two dissociations depending on the pH.

A diprotic acid is represented by H2A.

Dissociation of a diprotic acid does not happen all at once.

It has two steps of dissociation and each dissociation step has its own Ka value. The step 1 and step 2 Ka values are designated Ka1 and Ka2 respectively.

Back to the question,

At equilibrium,

Step 1

pKa1 = - log (Ka1) = - log (2.27 * 10^-4) = 3.64

Step 2

pKa2 = - log (Ka2) = - log (4.00 x 10^-11) = 10.40

The pH of H2A is given by

-log (molar concentration)

pH = - log (0.103) = - (-0.987) = 0.987. Approximately equals to 1