Question

Which of the following bases would you expect to have the strongest conjugate acid?

Group of answer choices

A.) PO4 3- (phosphate ion)
B.) HPO4 2- (hydrogen phosphate ion)
C.) H2PO 4- (dihydrogen phosphate ion)
D.) CO3 2- (Carbonate ion)

Answer 1

C.) H₂PO₄⁻ (dihydrogen phosphate ion)

Step-by-step explanation:

Acid-base reactions are defined by the exchange of a hydrogen ion.

Acid-Base Pairs

All acids and bases have conjugate pairs. A base's conjugate acid is the acid that is the product of the base. For example, take the conjugate pair NH₃ and NH₄⁺.

• NH₃ + H₂O ⇄ NH₄⁺

Since NH₄⁺ is the product of NH₃, they are a conjugate pair. In this reaction, NH₃ is the base and NH₄⁺ is the conjugate acid. Bases will have one less hydrogen ion than their conjugate acid.

Strength of Acids

In order to find the conjugate acids, we can add one hydrogen ion to each base.

• PO₄³⁻ → HPO₄²⁻
• HPO₄²⁻ → H₂PO₄⁻
• H₂PO₄⁻ → H₃PO₄
• CO₃²⁻ → HCO₃⁻

If you remember how polyprotic acids work, the original acid (the acid that has not been deprotonated) is always the strongest. Since H₃PO₄ is the only original acid, it is the strongest. This means that H₃PO₄ will more readily donate a hydrogen ion than any of the other conjugate acids. It is also helpful to know that weak bases have strong conjugate acids. H₂PO₄⁻ is a weak base, so its conjugate acid is strong. This means that H₂PO₄⁻ will have the strongest conjugate acid.