For an Acid That Undergoes This Reaction:
HA ↔ H⁺ \[HA]
B) [H⁺][HA]\[A ⁻ ]
C) [HA][A ⁻ ]\[H⁺]
D)

For an acid that undergoes this reaction:
HA ↔ H⁺ + A ⁻
K_a =

A) [H⁺][A ⁻ ] \[HA]
B) [H⁺][HA]\[A ⁻ ]
C) [HA][A ⁻ ]\[H⁺]
D) [A ⁻ ]\[HA][H⁺]
E) [H⁺]\[HA][A ⁻ ]