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 acid	HCl
hydrobromic acid	HBr
hydroiodic acid	HI
sulfuric acid	H2SO4
nitric acid	HNO3
perchloric acid	HClO4

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

Copyright © 1997-2001 by Fred Senese
Comments & questions to senese@antoine.frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101-hidden/acidbase/faq/strong-and-weak.shtml