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Lecture 2

CHEM 272 Lecture Notes - Lecture 2: Equivalence Point, Methyl Red, Acid Dissociation Constant


Department
Chemistry and Biochemistry
Course Code
CHEM 272
Professor
julin
Lecture
2

Page:
of 1
Jessi Schillinger
Chem272 8222
Discussion
1. The equivalence point for a strong base/strong acid titration is a pH of 7. This is
when the concentration of acid = the concentration of base. The ideal indicator
would be one that changes color before and after this equivalence point. When a
base is being titrated with an acid, the color change will occur when the In- to HIn
ratio is 10:1. Once the indicator changes color, the equivalence point has been
reached. The indicators pKa is best when equal to the pH at the equivalence
point. Terrapin yellow has the pKa of 7.4 which is closest to 7.
2. The average pH found for sample 1, the most acidic, was 4.4. This can be used to
find the concentration of H+. 10^-pH = 3.98x10^-5. The indicator that
corresponded to that was methyl red with a pKa of 5.0. Ka= 10^-pKA = 10^-5
HIn H+ In-
1.0x10^-6 3.98x10^-5 0
-x +x +x
1.0x10^-6 –x 3.98x10^-5+x x
Solve for x…
x=7.5x10^-7
x= moles so in the In- form, since x does not equal zero, there is still
some small amount in it.
The HIn form of the indicator is not the only form present in the most acidic solution,
there is always a possibility of a breakdown of the solution to make pH more basic. In the
Henderson-hasselbach equation as well, using the equation gives a value of 0, and this
cannot be used in the log formula so there must be some other basic form remaining and
this forces us to use the simple equation.