Which of the following species formed in acid base reactions can act as Bronsted buses as well as Bronsted acids?

Options

A. `[Fe(H_(2)O)_(6)]^(2+)`
B. `HPO_(4)^(2-)`
C. `HSO_(4)^(-)`
D. `NH_(4)^(+)`

Answer

To determine which species formed in acid-base reactions can act as both Bronsted acids and Bronsted bases, we need to understand the definitions of Bronsted acids and bases:

1. **Bronsted Acid**: A substance that can donate a proton (H⁺).
2. **Bronsted Base**: A substance that can accept a proton (H⁺).

Now, let's analyze the given options:

1. **[Fe(H₂O)₆]²⁺**: This complex can donate protons, acting as a Bronsted acid, but it does not have the ability to accept protons, so it cannot act as a Bronsted base.

2. **HPO₄²⁻**: This species can both donate a proton (acting as a Bronsted acid) and accept a proton (acting as a Bronsted base). Therefore, it qualifies as both a Bronsted acid and a Bronsted base.

3. **HSO₄⁻**: This ion can donate a proton (acting as a Bronsted acid) but does not accept protons, so it cannot act as a Bronsted base.

4. **NH₄⁺**: This ion can donate a proton (acting as a Bronsted acid) but cannot accept protons, thus it cannot act as a Bronsted base.

From this analysis, we conclude that the only species that can act as both a Bronsted acid and a Bronsted base is **HPO₄²⁻**.

### Final Answer:
**HPO₄²⁻** is the species that can act as both a Bronsted acid and a Bronsted base.