What are pKa and pKb values?  How is the pK value of any compound calculated?

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The pKa and pKb values are measurements of the strengths of acids and bases respectively. The smaller the pKa value, the stronger the acid. Likewise, the smaller the pKb, the stronger the base.

The pKa value of an acid is the negative base-ten logarithm of the acid dissociation constant of a solution. Or,

 pKa = -log10 Ka

When an acid is dissolved in water, it undergoes an equilibrium reaction. This reaction can be represented as follows:

 HA + H2O ↔ H3O+ + A-

 The value of the equilibrium constant, Ka, can be calculated using the following equation:

Ka = [H3O+] [A-] / HA

Likewise, when a base dissolves in water, the following equilibrium equation results:

B + H2O  ↔ BH+ + OH-

The equilibrium constant of a base, Kb, is found using the following equation:

Kb = [BH+] [OH-] / B

Thus, the following equation is used to find the pKb of a base:

 pKb = -log Kb

 

 

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