Lab 8 - 1 Lab 8: Determination of the Dissociation Constant...

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Lab 8: Determination of the Dissociation Constant of a Weak Acid
Lab 8: Determination of the Dissociation Constant of a weak acid
Wendy Yang
Tarrant County College
Southeast Campus
Dr. Masadeh
Chem 1412L
30 October 2015

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Lab 8: Determination of the Dissociation Constant of a Weak Acid
Introduction:
Acids are chemical substances that donate hydrogen ions and bases are substances that
accept hydrogen ions. Acids and bases react with each other by transferring hydrogen ions
and/or electrons, depending on which theory is being applied. According to Bronsted-Lowry, an
acid is a chemical substance that accepts a proton, while a base is a chemical substance that
donates a proton (Kemp, Nelson 98).
Acid strength can be related to its ability to donate protons (Kemp, Nelson 115). In
calculations, the acid strength can be determined by the amount of hydronium ion formed in
water solution. For the most part, it is expressed by pH, which equals the –log[H
3
O + ]. The
equilibrium constant for an acid dissociation is represented by K
a
= [H
3
O
+
] [A
-
] / [HA], where
HA is the acid and A
-
is the dissociated acid (Kemp, Nelson 115). Moreover, pKa = -logKa. In
other words, pH = pKa – log [HA]/[A-] (Kemp, Nelson 116).
In this experiment, a standardized solution of sodium hydroxide is first created. It then
was used to titrate KHP to find the molarity of the create sodium hydroxide. Then the sodium
hydroxide solution was then used on an unknown acid. The pK
a
of an unknown acid is
determined by titrating it with NaOH. The data is then graphed showing the effects of pH levels
when a certain amount of NaOH volume is use to titrate the unknown acid. The graph then
shows the equivalence point. Half of the equivalence point is the half-equivalence point of the
titration. At half equivalence, the [A - ] = [HA], so they cancel out in the equation K
a
= [H
3
O + ] [A - ]
/ [HA], leaving K
a
= [H
3
O + ]. With pH being known, K
a
is found by [H
3
O + ] equaling 10 -pH ;
therefore, pK
a
= pH.

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Lab 8: Determination of the Dissociation Constant of a Weak Acid
Pre Lab Questions:
1. Bronsted-Lowry acid is a substance that acts as a proton donor, where as a Bronsted-
Lowry base is a substance that acts as a proton acceptor.