Titration Curves - MCAT Chemical and Physical Foundations of Biological Systems

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Question

A titration reaches its equivalence point. You measure the pH and find it to be 6.4. You were most likely working with which of the following sets of compounds:

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Answer

For a strong acids and strong base, the equivalence pH will be 7.0. We can see that our equivalence pH is less than 7.0, meaning that the acidic component is prevailing. Hence, we must be working with an acid that is stronger than our base. HNO3 is a strong acid, and NH3 is a weak base, meaning that the acid will prevail, and the pH will be acidic (<7.0).

The other answer choices show a strong acid and strong base (HCl and NaOH), a weak acid and strong base (HF and KOH, HCN and LiOH), and a weak acid and weak base (H2S and C5H8O2). Though the equivalence pH of a weak acid and weak base may not be 7.0, we do not have enough information to choose this answer. The best answer choice is HNO3 and NH3

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