Final answer:
The equilibrium constant Kb for the conjugate base A- is represented by option 4: H2O + A- ⇌ HA + OH-. This equation reflects the hydrolysis of the base in water, forming HA and OH- ions.
Step-by-step explanation:
The equilibrium constant Kb for the weak acid's conjugate base, A-, corresponds to the reverse reaction of the acid ionization, which describes the reaction of A- with water to form HA and hydroxide ions (OH-). The correct equilibrium for the Kb of A- is given by equation 4, which is:
H2O + A- ⇌ HA + OH-
This represents the hydrolysis of the conjugate base A- and shows the formation of OH-. The base ionization constant, Kb, can be determined experimentally, much like the acid ionization constant, Ka, however for the conjugate base it uses the concentrations of OH- and HA relative to A- in the equilibrium mixture.
When a weak acid reacts with water, it forms its conjugate base (A-) and hydronium ions (H3O+). This is represented by the equation: HA + H2O ⇌ H3O+ + A-. In this equation, the equilibrium constant Ka is for the forward reaction, while the equilibrium constant Kb is for the reverse reaction. Therefore, option 4) H2O + A- ⇌ HA + OH- represents the equilibrium constant Kb for A-.