Which of the following statements correctly defines a weak acid?

What is the correct equilibrium expression for the dissociation of a weak acid HA in water?

If the pH of a solution is 3.5, what is the concentration of hydronium ions, [H₃O⁺]?

Given a weak acid HA with an initial concentration of 0.1 M and a K_a of 1.8 x 10^-5, use an ICE chart to find the equilibrium concentration of H₃O⁺.

Given a weak acid HA with an initial concentration of 0.05 M and a K_a of 2.5 x 10^-5, use an ICE chart to find the equilibrium concentration of A^-.