MCAT General Chemistry Question 291: Answer and Explanation

Home > MCAT Test > MCAT general chemistry practice tests

Test Information

Use your browser's back button to return to your test results.
Do more MCAT general chemistry practice tests.

Question: 291

4. In the dissociation of phosphoric acid, the trend K_a1 > K_a2 > K_a3 is predominantly due to:

A. an equilibrium shift towards the reactants side in Reactions 2 and 3 due to the release of H⁺ in Reaction 1.
B. a smaller radius in the H⁺ liberated in Reaction 1 compared to that in Reactions 2 and 3.
C. a slower rate of reaction after subsequent ionizations.
D. an increasing influence of the anion after subsequent ionizations.

Correct Answer: D

Explanation:

D Generation of H⁺ in Reaction 1 is coupled with a release of H₂PO₄^-. Both the product and reactant sides of Reaction 2 are increased proportionally, causing no shift in equilibrium (eliminate choice A). Atomic radius is a function of an atom's position in the periodic table. Thus, the radius of H⁺ is the same in all three reactions, eliminating choice B. Equilibrium constants have no relationship to reaction rates, so choice C can be eliminated. The K_a values progressively decrease when removing a proton from a polyprotic acid because it is more difficult to remove a proton from an anion compared to a neutral molecule. In subsequent ionizations, the anion becomes more negative, resulting in greater difficulty liberating a positively charged H⁺ ion.

Previous Next