When biologists study the chemical reactions that occur in living systems, they often use buffer solutions to mimic the pH and osmotic pressure inside a living cell. A common buffer solution consists of dihydrogen phosphate (H₂PO₄⁻) and its conjugate base. This acid-base pair undergoes the equilibrium reaction shown here:
H₂PO₄⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HPO₄²⁻(aq)
K = 6.31 × 10⁻⁸
Write an equilibrium expression for this equation.