MadSci Network: Chemistry

Re: what is the difference between strong acid and concentrated acid?

Date: Fri Mar 2 14:49:09 2001
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 983381629.Ch

what is the difference between strong acid and concentrated acid?
A strong acid is one that is completely dissociated in a particular solvent, when the concentration is not too high:

HA + S ® A- + HS+

How strong this needs to be will vary from solvent to solvent, but we normally think of a strong acid as one that is completely dissociated in water.

We need to be careful here: phosphoric acid (H3PO4) is a strong acid, but does not lose all three protons in aqueous solution.

H3PO4 + 2H2O ® HPO4-- + 2H3O+

H3PO4 normally behaves in water as a dibasic (2-proton) acid toward weak bases. It takes a pretty strong base, like hydroxide, to remove the third proton.

A weak acid is one that is only partly dissociated, no matter what the concentration. For example, acetic acid (the active ingredient in vinegar) is only about 1% dissociated in water, even in dilute solutions.

A concentrated acid is an acid which is either pure (no solvent) or has a high concentration. Glacial acetic acid is a concentrated weak acid, and industrial sulfuric acid (about 98% by weight, the other 2% is water) is a concentrated strong acid.

Even strong acids are not completely dissociated when the concentration is too high: concentrated hydrochloric acid, even though it is only 12-13 moles per liter in water, is concentrated enough that there are irritating fumes of HCl gas above the acid. If you dilute the acid to 6 moles per liter, the fumes are no longer noticeable because the HCl is completely dissociated.

Dan Berger
Bluffton College

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