Final answer:
The Kb for A- represents the base dissociation constant when A- reacts with water, producing HA and OH-. It is calculated using the expression Kb = [HA][OH-]/[A-]. The strength of the base is indicated by the magnitude of Kb, with higher values representing stronger bases.
Step-by-step explanation:
The equilibrium constant Kb for the base A- comes from the reaction in which A- reacts with water to produce HA and OH-. The reaction is represented as A- + H2O ⇌ HA + OH-. The equilibrium expression for this reaction, which defines Kb, would be:
Kb = [HA][OH-]/[A-].
For a weak acid HA, with a given acid ionization constant (Ka), its conjugate base A- will have a corresponding base dissociation constant (Kb). The two constants are related through the autoionization constant of water (Kw), so that Kb = Kw/Ka. To find the pH and equilibrium concentration of a weak acid solution, one must use the Ka and an ICE table to solve for the hydronium ion concentration [H3O+], and then use the negative logarithm to find pH.
Given the equilibrium expression for the acid in question: HA ⇌ H+ + A-, we associate the equilibrium constant expression Ka = [H+][A-]/[HA]. However, for the base providing the Kb, we consider the reaction of its conjugate base with water: A- + H2O ⇌ HA + OH-, with Kb = [HA][OH-]/[A-]. For a big explanation part, you must know that the value of Kb is indicative of the strength of the base: the higher the Kb, the stronger the base.