Question

consider the reaction below.
h₂po₄⁻ + h₂o → h₃o⁺ + hpo₄²⁻
which of the following is a base–conjugate acid pair?
∘ h₂o and h₃o⁺
∘ h₂o and h₂po₄⁻
∘ h₂po₄⁻ and hpo₄²⁻
∘ h₂po₄⁻ and h₃o⁺

Explanation:

To identify a base - conjugate acid pair, we use the Bronsted - Lowry definition. A base is a substance that accepts a proton ($\ce{H^{+}}$), and its conjugate acid is the substance formed when the base gains a proton.

• For the pair $\ce{H_{2}O}$ and $\ce{H_{3}O^{+}}$: In the reaction $\ce{H_{2}PO_{4}^{-}+H_{2}O

ightarrow H_{3}O^{+}+HPO_{4}^{2 - }}$, $\ce{H_{2}O}$ accepts a proton ($\ce{H^{+}}$) to form $\ce{H_{3}O^{+}}$. So $\ce{H_{2}O}$ is a base and $\ce{H_{3}O^{+}}$ is its conjugate acid.

• For the pair $\ce{H_{2}O}$ and $\ce{H_{2}PO_{4}^{-}}$: $\ce{H_{2}O}$ and $\ce{H_{2}PO_{4}^{-}}$ do not have a proton - accepting - and - forming relationship. $\ce{H_{2}PO_{4}^{-}}$ donates a proton to form $\ce{HPO_{4}^{2 - }}$, and $\ce{H_{2}O}$ accepts a proton from $\ce{H_{2}PO_{4}^{-}}$, but they are not a base - conjugate acid pair.
• For the pair $\ce{H_{2}PO_{4}^{-}}$ and $\ce{HPO_{4}^{2 - }}$: $\ce{H_{2}PO_{4}^{-}}$ donates a proton to form $\ce{HPO_{4}^{2 - }}$, so they are an acid - conjugate base pair, not a base - conjugate acid pair.
• For the pair $\ce{H_{2}PO_{4}^{-}}$ and $\ce{H_{3}O^{+}}$: $\ce{H_{2}PO_{4}^{-}}$ donates a proton and $\ce{H_{3}O^{+}}$ is formed from $\ce{H_{2}O}$ accepting a proton, so they are not a base - conjugate acid pair.

Answer:

A. $\ce{H_{2}O}$ and $\ce{H_{3}O^{+}}$