To identify the base-conjugate acid pair in the given reaction, it's essential to understand the concept of acid-base conjugation. In a chemical reaction involving acids and bases:
- An acid loses a proton (H⁺) to form its conjugate base.
- A base gains a proton (H⁺) to form its conjugate acid.
Let's analyze the given reaction:
[tex]\[ \text{H}_2\text{PO}_4^- + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{HPO}_4^{2-} \][/tex]
From this reaction:
- Di-hydrogen phosphate ([tex]\(\text{H}_2\text{PO}_4^-\)[/tex]) is reacting with water ([tex]\(\text{H}_2\text{O}\)[/tex]).
- The product formed includes hydronium ([tex]\(\text{H}_3\text{O}^+\)[/tex]) and hydrogen phosphate ([tex]\(\text{HPO}_4^{2-}\)[/tex]).
Let's identify what gains or loses a proton:
- [tex]\(\text{H}_2\text{PO}_4^-\)[/tex] loses a proton (H⁺) to form [tex]\(\text{HPO}_4^{2-}\)[/tex]. Therefore, [tex]\(\text{H}_2\text{PO}_4^-\)[/tex] is acting as an acid, and [tex]\(\text{HPO}_4^{2-}\)[/tex] is its conjugate base.
- [tex]\(\text{H}_2\text{O}\)[/tex] gains a proton (H⁺) to form [tex]\(\text{H}_3\text{O}^+\)[/tex]. Thus, [tex]\(\text{H}_2\text{O}\)[/tex] is acting as a base, and [tex]\(\text{H}_3\text{O}^+\)[/tex] is its conjugate acid.
Considering the options provided:
1. [tex]\( \text{H}_2\text{O} \)[/tex] and [tex]\( \text{H}_3\text{O}^+ \)[/tex] form a conjugate base-acid pair.
2. [tex]\( \text{H}_2\text{O} \)[/tex] and [tex]\( \text{H}_2\text{PO}_4^- \)[/tex] do not form a conjugate pair.
3. [tex]\( \text{H}_2\text{PO}_4^- \)[/tex] and [tex]\( \text{HPO}_4^{2-} \)[/tex] form a conjugate acid-base pair.
4. [tex]\( \text{H}_2\text{PO}_4^- \)[/tex] and [tex]\( \text{H}_3\text{O}^+ \)[/tex] do not form a conjugate pair.
The correct pair that forms a base-conjugate acid pair in the given reaction is:
[tex]\[ \text{H}_2\text{PO}_4^- \text{ and } \text{HPO}_4^{2-} \][/tex]
Therefore, the answer is:
[tex]\[ \boxed{3} \][/tex]
