Which of the following reactions best represents the reaction between H3PO4
and water?
A) H3PO4 + H2O ⇌ H2PO4
- + H2O
B) H3PO4 + H2O ⇌ H2PO4
- + H3O+
C) H3PO4 + 3 H2O → PO4 3- + 3 H2O
D) H3PO4 + 3 H2O → PO4 3- + 3 H3O+

Option B,

Explanation:

In this reaction, a weak acid is reacting with  water. Thus, water is this case will act as a proton acceptor or a base as well as an acid. Hence water will be amphiprotic for this chemical process and can donate as well accept as a proton. Now when weak acid such as phosphoric acid loses a hydrogen ion it forms a weak conjugate base ie. H2PO4^-. Water being a weak base shall accept the proton and forms hydronium ion i.e H3O^+

The dihydrogen phosphate ion reacts with water:

H2PO4^- + H2O <----> HPO4^2- + H3O^+

After some time a proton is again transferred to the H2O molecule to produce  phosphate ion

HPO4^2- + H2O <----> H3O^+ + PO4^3-