Which ONE of the following pairs represents the conjugate acid and the conjugate base of HPO₄²⁻ ? CONJUGATE ACID CONJUGATE BASE A PO₄³⁻ B H₂PO₄⁻ C HPO₄²⁻ D PO₄²⁻ - NSC Physical Sciences - Question 1 - 2017 - Paper 2

To determine the conjugate acid and base pair of HPO₄²⁻, we first need to understand the definitions:

• A conjugate acid is formed when a base gains a proton (H⁺).
• A conjugate base is formed when an acid loses a proton.

Starting with HPO₄²⁻, if we add a proton (H⁺), it becomes H₂PO₄⁻. Therefore, H₂PO₄⁻ is the conjugate acid of HPO₄²⁻.

Conversely, if HPO₄²⁻ loses a proton (H⁺), it becomes PO₄³⁻, which is its conjugate base.

Thus, the correct conjugate acid-base pairs relevant to HPO₄²⁻ would be:

• Conjugate Acid: H₂PO₄⁻
• Conjugate Base: PO₄³⁻.

From the options provided, option B (H₂PO₄⁻ as the conjugate acid) correctly represents the conjugate acid of HPO₄²⁻.