Define Kb and pKb and explain how they relate to base strength.

Define Kb and pKb and explain how they relate to base strength.

• A. Kb is the acid dissociation constant; larger Kb means stronger acid.
• B. pKb is the concentration of base.
• C. Kb is the base dissociation constant; larger Kb means stronger base.
• D. pKb equals pH.

Answer: (C)

This question tests how base strength is quantified with dissociation constants. For a base B reacting with water, the reaction B + H2O ⇌ BH+ + OH− has the base dissociation constant Kb = [BH+][OH−]/[B]. A larger Kb means the equilibrium lies further to the right, producing more hydroxide in solution, so the base is stronger. The related quantity pKb is defined as pKb = −log10(Kb). It converts the constant into a more convenient, smaller-number scale: a stronger base has a larger Kb and thus a smaller pKb. Remember also that the conjugate acid–base pair relation Ka × Kb = Kw makes pKa + pKb ≈ pKw (about 14 at 25°C), reinforcing how changes in base strength mirror changes in acid strength.

This question tests how base strength is quantified with dissociation constants. For a base B reacting with water, the reaction B + H2O ⇌ BH+ + OH− has the base dissociation constant Kb = [BH+][OH−]/[B]. A larger Kb means the equilibrium lies further to the right, producing more hydroxide in solution, so the base is stronger. The related quantity pKb is defined as pKb = −log10(Kb). It converts the constant into a more convenient, smaller-number scale: a stronger base has a larger Kb and thus a smaller pKb. Remember also that the conjugate acid–base pair relation Ka × Kb = Kw makes pKa + pKb ≈ pKw (about 14 at 25°C), reinforcing how changes in base strength mirror changes in acid strength.