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pKa Table

Article ID: 109
Last updated: 25 Jan, 2012
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The pKa table is a crucial tool for any organic chemist when ranking the strength of acids or bases.

Check out this very comprehensive pKa table

The pKa values given in this table represent the -log[Ka] of an acid (pKa = -log[Ka]).  The Ka describes the extent to which a specific acid dissociates to its conjugate base.  A large Ka represents a strong acid that dissociates easily.  SO, mathematically a lower pKa value represents a stronger acid.

If two acids are in equilibrium with a >= 3 pKa unit difference between them, the lower pKa acid will almost completely dissociate.  Organic chemists also categorize compounds by their pKa value into four groups: strong acid, weak acid, very weak acid, extremely weak acid.  Those groups are described below:

Strong acid: pKa < H3O+

Weak acid: pKa > H3O+ and < H2O

Very weak acid: pKa > H2O and < NH3

Extremely weak acid: pKa > NH3

In your own words state why: a) HF has a higher pKa than HI, b) H2S has a lower pKa than H2O, and c) explaining the pKa values of H2S and HF are difficult to do.

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Department of Chemistry & Chemical Biology