Question

Consider a triprotic acid like phosphoric acid or citric acid. Which expression correctly describes the relative magnitudes of the three equilibrium constants?

a. There is no generally-applicable order.
b. pKa1 > pKa2 > pKa3
c. pka1 < pKa2 < pKa3
d. Ka1 < Ka2 < Ka3

Answer 1

The correct expression that describes the relative magnitudes of the three equilibrium constants for a triprotic acid is 'pKa1 < pKa2 < pKa3', because the first proton dissociation has the strongest acid with the highest Ka and the strongest acid has the smallest pKa.

Step-by-step explanation:

The question refers to the correct expression that describes the relative magnitudes of the three equilibrium constants for a triprotic acid such as phosphoric acid or citric acid. The correct answer is c. pKa1 < pKa2 < pKa3.

This is because for polyprotic acids, the first dissociation step usually has the strongest acid (highest acid ionization constant, Ka), followed by weaker acids (lower acid ionization constants, Ka) for the subsequent dissociation steps.

Consequently, the pKa values, which are the negative logarithms of the Ka values, increase with each step, indicating that the acidic strength diminishes for each subsequently removed proton.