Question

How does H₂PO₄⁻ act as both an acid and a base?

a) H₂PO₄⁻ + H₂O ⟶ H₃O⁺ + HPO₄²⁻
b) H₂PO₄⁻ ⟶ HPO₄²⁻ + H⁺
c) H₂PO₄⁻ + OH⁻ ⟶ PO₄³⁻ + H₂O
d) H₂PO₄⁻ ⟶ H₃PO₄ + e⁻

Answer 1

c. H₂PO₄⁻ is amphoteric because it can act as both an acid and a base, donating a proton in its reaction with OH⁻ or accepting a proton in its reaction with HBr.

Step-by-step explanation:

c. H₂PO₄⁻ acts as both an acid and a base due to its ability to donate and accept protons, respectively. This characteristic makes it amphoteric. For instance, when acting as a base, H₂PO₄⁻ can accept a proton from an acid such as HBr, forming H₃PO₄ and releasing a Br⁻ ion:

(a) H₂PO₄⁻ (aq) + HBr(aq) = H₃PO₄ (aq) + Br⁻(aq)

Conversely, as an acid, H₂PO₄⁻ can donate a proton to a base such as OH⁻, forming HPO₄²⁻ and water:

(b) H₂PO₄⁻(aq) + OH⁻(aq) ⇒ HPO₄²⁻(aq) + H₂O(l)