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Home :FAQ :Acids and basesPrint | Comment
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Can you classify acids as strong or weak from their formulas alone?

I want to know how to distinguish a strong acid and a weak acid at glance, without knowing if it reacts with water or base. I also want to do this for strong and weak bases. For example, is CH4 a strong or weak acid; is C2H5OH a strong or weak base?
Vaneswwei, University of California at Santa Cruz

Here are some very simple guidelines to classify compounds as strong or weak acids and bases. But they're just that: guidelines. Apply them with caution and as always, common sense and experimental evidence prevails.

There are very few strong acids in aqueous solution. In fact, only the following six are common:

hydrochloric acidHCl
hydrobromic acidHBr
hydroiodic acidHI
sulfuric acidH2SO4
nitric acidHNO3
perchloric acidHClO4

If the compound is an acid, and it is not on this list, you are usually safe in assuming it's a weak acid in water. (Your example, CH4, is a covalent compound. It would have negligible acidity in water.)

Strong bases are also very rare. The most common strong bases are the alkali metal hydroxides and the alkaline earth metal hydroxides (except Be(OH)2). If you have a base that is not one of these in water, you can tentatively assume that it's a weak base.

Author: Fred Senese senese@antoine.frostburg.edu



General Chemistry Online! Can you classify acids as strong or weak from their formulas alone?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/strong-and-weak.shtml