How the concentration of a weak acid affects its dissociation in water?

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Posted on : 03-12-2013 | By : My Study Coach | In : Improve Your Concentration
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Again, how the concentration of a weak acid in an aqueous solution affects its dissociation and why? This is the university question I can hardly answer but need detailed (good) explanation. Please, I need your help, and, thanx in advance! ;)

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Comments (2)

As a weak acid is diluted more and more, then it dissociates to a greater and greater extent. You can explain it using Ka calculations or by using Le Chatelier’s principle.

HA < ==> H+ + A- ….. Ka is small

By adding more water, you are decreasing the concentration of both H+ and A-, and have disturbed the equilibrium. Le Chaelier’s principle says that the equilibrium will shift to the right in order to being the concentrations back closer to what they were initially. Therefore, more HA will dissociate to make additional H+ and A-. With “infinite” dilution, 100% of the weak acid will dissociate.

============= Follow up ==============

Roger has given you a nice explanation of what a weak acid is, but since you ask about weak acids, we’ll assume you know that. Roger, though, seems to have missed the point. In your case, you’re not adding or removing H+, you are simply diluting the solution by adding water. As water is added, not H+ are added or removed, but there will be a shift in the position of the equilibrium. Adding water decreases the concentration of the H+ and so the equilibrium shift to the right to bring it back up, and that causes some undissociated HA to dissociate. By adding enough water you can make force all of the acid to dissociate.

By the way, the word is “dissociate’, not “disassociate”.

The first thing to understand is what an acid is. It is any water soluble substance which produces Hydrogen ions.

The next thing to understand is what a weak acid is. Unlike strong acids, weak ones generate only a few Hydrogen ions. Most of the weak acid remains as an inert molecule and forms no ions. Also, the ratio of ions to molecules is fixed and this establishes an equilibrium. If some ions are removed, a few weak acid molecules form ions to replace them. This will continue until all the weak acid has formed ions. because the weak acid maintains a constant concentration of ions, the pH remains the same. This is what buffering is.

HA -> H(+) + A(-)

The equation above describes the disassociation of an acid into a pair of ions. Adding either molecules or ions will increase the products on the other side of the equation. This re establishes the equilibrium between ions and molecules. If more weak acid is added, it will increase the number of Hydrogen ions and the pH will fall. If the acid is removed, the pH rise. PH decreases with increasing acidity.

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