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Acidity ladder

Rather than memorising absolute values, having a good knowledge of the general order of pKas can often be good enough, more than enough in fact..... and fortunately, the most important pKa values can often be rounded off to the nearest 5....

Remember that the weakest acids have the highest pKas and are at the top, the strongest acids have the lowest pKas and they are at the bottom of this ladder.

When using the "ladder" make sure that you are considering the right acid (especially for systems like water and amines) since they have two equilibria and both appear on the ladder.

The ladder shows the approximate pKa of the corresponding acid, for example, the pKa of water is about 15 (actual value = 14) corresponding to the reaction H2O <=> H+ + HO-

© Dr. Ian Hunt, Department of Chemistry, University of Calgary