How to Calculate the K Value on a Titration Graph

In a titration experiment, the student pours the known solution from a buret.
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The K value on a titration graph is either the Ka or the Kb. The Ka is the acid dissociation constant and the Kb is the base dissociation constant. The titration graph represent the various pH levels that occur when a solution of an unknown pH is being poured into a solution with a known pH. The pH of the solution is on the y-axis of the titration graph and the volume of the solution is on the x-axis of the graph. It is useful to known how to calculate the K value on a titration graph, because such a process is used in most chemistry lab experiments with acids and bases.

    Examine the structure of the titration graph. The titration graph typically rises horizontally, vertically, and then horizontally again. The center of the vertical portion of the graph is the equivalence point, or the point at which the pH of the unknown solution begins to change. For example, if the unknown solution is a strong acid, and the known solution is a strong base, then the equivalence point will occur at a pH of 7 because after 7, the pH of the acidic solution will become basic.

    Use the Henderson-Hasselbalch equation to understand the value of the pKa at the equivalence point. The pKa of the solution is the negative logarithm of the Ka. The Henderson-Hasselbalch equation is pH = pKa + log ([base] / [acid]). At the equivalence point, the concentrations of the base and the acid are equal. The log of 1 is equal to 0. Therefore, pH = pKa. So at a pH of 7, the pKa is equal to 7.

    Use the equation for the pKa to determine the value of the Ka. The equation for the pKa is pKa = - log(Ka). Therefore, 10 ^ (-pKa) = Ka. If the pKa is 7, then 10 ^ -7 = 1.0 x 10 ^ -7. The value of Ka on the titration graph is Ka = 1.0 x 10 ^ -7.

    Things You'll Need

    • Chemistry book
    • Calculator

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