Which of the following can act both as a Bronsted acid and a Bronsted base ?
(a)`HCO_3^-` , (b) `O^2^-`, (c) `NH_3` , (d) `H_2O` , (e) `HS^-`

Answer

To determine which of the given species can act both as a Bronsted acid and a Bronsted base, we need to understand the definitions of Bronsted acids and bases:

- **Bronsted Acid**: A substance that donates a proton (H⁺).
- **Bronsted Base**: A substance that accepts a proton (H⁺).

Now, let's analyze each option step by step:

### Step 1: Analyze `HCO₃⁻` (Bicarbonate Ion)
- **As a Bronsted Acid**: It can donate a proton to form `CO₃²⁻` (carbonate ion).
\[ HCO₃⁻ \rightarrow CO₃²⁻ + H⁺ \]
- **As a Bronsted Base**: It can accept a proton to form `H₂CO₃` (carbonic acid).
\[ HCO₃⁻ + H⁺ \rightarrow H₂CO₃ \]
- **Conclusion**: `HCO₃⁻` can act as both a Bronsted acid and a Bronsted base.

### Step 2: Analyze `O²⁻` (Oxide Ion)
- **As a Bronsted Acid**: It does not have a proton to donate.
- **As a Bronsted Base**: It can accept a proton to form `OH⁻` (hydroxide ion).
\[ O²⁻ + H⁺ \rightarrow OH⁻ \]
- **Conclusion**: `O²⁻` can only act as a Bronsted base, not as an acid.

### Step 3: Analyze `NH₃` (Ammonia)
- **As a Bronsted Acid**: It can donate a proton to form `NH₂⁻` (amide ion).
\[ NH₃ \rightarrow NH₂⁻ + H⁺ \]
- **As a Bronsted Base**: It can accept a proton to form `NH₄⁺` (ammonium ion).
\[ NH₃ + H⁺ \rightarrow NH₄⁺ \]
- **Conclusion**: `NH₃` can act as both a Bronsted acid and a Bronsted base.

### Step 4: Analyze `H₂O` (Water)
- **As a Bronsted Acid**: It can donate a proton to form `OH⁻` (hydroxide ion).
\[ H₂O \rightarrow OH⁻ + H⁺ \]
- **As a Bronsted Base**: It can accept a proton to form `H₃O⁺` (hydronium ion).
\[ H₂O + H⁺ \rightarrow H₃O⁺ \]
- **Conclusion**: `H₂O` can act as both a Bronsted acid and a Bronsted base.

### Step 5: Analyze `HS⁻` (Hydrosulfide Ion)
- **As a Bronsted Acid**: It can donate a proton to form `S²⁻` (sulfide ion).
\[ HS⁻ \rightarrow S²⁻ + H⁺ \]
- **As a Bronsted Base**: It can accept a proton to form `H₂S` (hydrogen sulfide).
\[ HS⁻ + H⁺ \rightarrow H₂S \]
- **Conclusion**: `HS⁻` can act as both a Bronsted acid and a Bronsted base.

### Final Conclusion
The species that can act both as a Bronsted acid and a Bronsted base are:
- `HCO₃⁻`
- `NH₃`
- `H₂O`
- `HS⁻`

The only species that cannot act as both is `O²⁻`, which can only act as a Bronsted base.

### Summary of Answers:
- **Answer**: (a) `HCO₃⁻`, (c) `NH₃`, (d) `H₂O`, (e) `HS⁻` can act as both Bronsted acid and base.