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Unformatted text preview: e is the Henderson - Hasselbalch equation, the equation that describes the pH when a weak
acid and its conjugate base are both present in solution. The word "buffer" is used to describe this
solution, since it has the ability of being able to resist dramatic changes in pH upon addition of either a
strong acid or a strong base. The conjugate base A- will react with any strong acid added while the weak
acid HA will react with any strong base added.
At half equivalence point, the mmoles of base added are only half the mmoles of acid originally present,
the concentrations of the conjugate base and the remaining weak acid are the same. Therefore, at halfequivalence;
pH = pK a
(c) equivalence point - At this point, only the conjugate base of the weak acid needs our attention. Thus,
at the equivalence point, we are simply dealing with a weak base solution: (d) Strong base region (after equivalence point) - in this region, one can predict the pH by simply taking
into account the amount of excess base. At 25o C;
pH = 14 - pOH
= 14 - log [ ((mmoles of base) - (mmoles of acid)) / (vol. acid + vol. base) ]
One can imagine a similar scenario for a weak base - strong acid titration.
The above illustrates a significant amount of approximations and for the purposes of a General Chemistry course, the approach
that we have described here is valid. There is a correct and exact way of treating acid-base titration curves. This correct way
makes use of simultaneous equations derived from both mass and charge balance equations in addition to the equilibrium
expressions. This quantitative treatment is essential for correctly predicting a titration curve in its entirety. This treatment,
however, is beyond the scope of this course. Go back to Chem 002 syllabus page...
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This note was uploaded on 09/29/2013 for the course CHEM 14CL taught by Professor Henary during the Fall '08 term at UCLA.
- Fall '08