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Name: Livia
Status: student
Grade: 12
Country: Canada
Date: Fall 2009


Question:
Hello there, I am simply wondering if a weak acid will neutralize the same amount of strong base as a strong acid with the same concentration? We had a question like this in Chemistry, asking how much 0.1 mol/L ethanoic acid (CH3COOH) is needed to neutralize 20mL of NaOH when 25mL of 0.1 mol/L HCl is needed to neutralize the same amount of NaOH.



Replies:
Yes.

In the case of neutralizing sodium hydroxide with acetic (systematically, "ethanoic") acid, you will make sodium acetate. When neutralizing sodium hydroxide with hydrochloric acid, you will make sodium chloride. The sodium acetate is itself a weak base, unlike sodium chloride. It will take 25 mL of acid solution to reach the endpoint in either case.

I am not terrifically comfortable with the term "neutralize." At their endpoints, the two solutions will not have exactly the same pH. You would need to add more acetic acid to get the pH down to the level of the endpoint with hydrochloric acid.

Richard Barrans, Ph.D., M.Ed.
University of Wyoming


Livia,

Remember that the only difference between a weak and strong acid is that a weak acid forms an equilibrium with its ions whereas a strong acid fully dissociates to its ions.

Keeping this in mind then, if a weak acid were to partially dissociate (form an equilibrium), then any H+ (or H3O+) present in solution will react with the base. But when it does so, according to the principles described by Le Chatelier, more of the acid will dissociate to form H+, and this will react with additional base. This shifting of the equilibrium to form more H+ will continue until all of the weak base is consumed.

This means then that in a reaction of an acid, HA, it doesn't matter if the HA dissociates or equilibrates, all the H will be reacted.

Greg (Roberto Gregorius)
Canisius College



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