Question

question 24 (1 point)
use the following information to answer the next question.
phosphoric acid, h₃po₄(aq), is a triprotic acid. it helps
in removing rust from iron, acidifying foods and
beverages. it causes lowering of bone density.
which of the following expressions correctly represents each of the ionizations that
take place in the aqueous solution of phosphoric acid?
options with ka expressions (omitted here for brevity as they are in the image)

Explanation:

Step1: Recall Triprotic Acid Ionization

A triprotic acid (like \( \ce{H3PO4} \)) ionizes in three steps. The first ionization: \( \ce{H3PO4 + H2O <=> H3O+ + H2PO4-} \). The acid dissociation constant \( K_{a1} \) is \( \frac{[\ce{H3O+}][\ce{H2PO4-}]}{[\ce{H3PO4}]} \).

Step2: Second Ionization Step

Second ionization: \( \ce{H2PO4- + H2O <=> H3O+ + HPO4^{2-}} \). So \( K_{a2} = \frac{[\ce{H3O+}][\ce{HPO4^{2-}}]}{[\ce{H2PO4-}]} \).

Step3: Third Ionization Step

Third ionization: \( \ce{HPO4^{2-} + H2O <=> H3O+ + PO4^{3-}} \). Thus \( K_{a3} = \frac{[\ce{H3O+}][\ce{PO4^{3-}}]}{[\ce{HPO4^{2-}}]} \).

Step4: Match with Options

Compare the derived \( K_{a1} \), \( K_{a2} \), \( K_{a3} \) with the options. The second option has \( K_{a1} = \frac{[\ce{H3O+}][\ce{H2PO4-}]}{[\ce{H3PO4}]} \), \( K_{a2} = \frac{[\ce{H3O+}][\ce{HPO4^{2-}}]}{[\ce{H2PO4-}]} \), \( K_{a3} = \frac{[\ce{H3O+}][\ce{PO4^{3-}}]}{[\ce{HPO4^{2-}}]} \), which matches the ionization steps.

Answer:

The second option (the one with \( K_{a_1} = \frac{[\ce{H3O+}][\ce{H2PO4^-}]}{[\ce{H3PO4}]} \), \( K_{a_2} = \frac{[\ce{H3O+}][\ce{HPO4^{2-}}]}{[\ce{H2PO4^-}]} \), \( K_{a_3} = \frac{[\ce{H3O+}][\ce{PO4^{3-}}]}{[\ce{HPO4^{2-}}]} \))