Q.The species: H2O, HCO3 –, HSO4 – and NH3 can act both as Bronsted acids and bases. For each case give the corresponding conjugate acid and conjugate base.

Concept: Brønsted–Lowry Conjugate Pairs

A Brønsted–Lowry acid donates a proton ($\text{H}^+$), leaving behind its conjugate base. A Brønsted–Lowry base accepts a proton, forming its conjugate acid. Amphoteric species can do both.

For each species, we remove one $\text{H}^+$ to find the conjugate base and add one $\text{H}^+$ to find the conjugate acid:

The Brønsted–Lowry theory defines acids as proton donors and bases as proton acceptors. What makes certain species particularly interesting is their ability to play both roles depending on the chemical environment—these are called amphoteric or amphiprotic species.

When a species acts as an acid, it donates a proton and leaves behind its conjugate base. When it acts as a base, it accepts a proton and becomes its conjugate acid. The relationship is simple:

Let's systematically work through each of the four species:

1. Water ($\text{H}_2\text{O}$)

Water is the classic amphoteric molecule.

As an acid: $\text{H}_2\text{O}$ donates a proton to form the hydroxide ion:

Conjugate base: $\text{OH}^-$

As a base: $\text{H}_2\text{O}$ accepts a proton to form the hydronium ion:

Conjugate acid: $\text{H}_3\text{O}^+$

2. Bicarbonate ion ($\text{HCO}_3^-$)

This ion sits in the middle of the carbonate system, making it naturally amphoteric.

As an acid: $\text{HCO}_3^-$ donates a proton to form carbonate:

Conjugate base: $\text{CO}_3^{2-}$

As a base: $\text{HCO}_3^-$ accepts a proton to form carbonic acid:

Conjugate acid: $\text{H}_2\text{CO}_3$

3. Hydrogen sulfate ion ($\text{HSO}_4^-$)

Similar to bicarbonate, this is the intermediate species in the sulfuric acid system.

As an acid: $\text{HSO}_4^-$ donates a proton to form sulfate:

Conjugate base: $\text{SO}_4^{2-}$

As a base: $\text{HSO}_4^-$ accepts a proton to form sulfuric acid:

Conjugate acid: $\text{H}_2\text{SO}_4$

4. Ammonia ($\text{NH}_3$)

Ammonia is typically thought of as a base, but it can act as an acid in the presence of very strong bases.

As a base: $\text{NH}_3$ accepts a proton to form ammonium:

Conjugate acid: $\text{NH}_4^+$ …