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ACT Question for August 19th

QUESTION

Although the pH (acidity) of pure water is approximately 7, the pH can change greatly if even a single drop of strong acid or strong base is added.  Compounds that can act as weak acids or bases will minimize this change in pH, and such compounds are called buffers.  Each buffer is most effective at a particular pH, referred to as the buffer's pK' value.

Study 1
A chemistry student performs 3 titrations - one of a carbonic acid solution, one with a phosphoric acid solution, and one with an ammonium solution.  In each case, NaOH is gradually added, and the resulting pH is measured after each addition.  The results are shown in Figure 1.  As expected, the pH changes more and more slowly until it equals the indicated pK' value. (For instance, the phosphoric acid curve is closest to horizontal when the pH reaches the pK' of phosphoric acid, 7.2.)  After this point, the pH changes more and more rapidly again.

Figure 1



From the slopes of the curves, the student also calculated how the buffering ability of each compound varied with pH.  The results are shown in Figure 2. Each curve reaches its maximum at the buffer's pK' value.

Figure 2


Study 2
The student performs a fourth titration, using the amino acid alanine.  This compound contains multiple groups that can act as acids or bases.  The results are shown in Figure 3.

Figure 3


In Figure 1, the titration curve for ammonium lies approximately:

ANSWER SELECTION

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Question ID: 5846