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MCAT Chemistry Question — Titration Curve

A protic acid is being titrated with sodium hydroxide (NaOH) when the pH of the solution stays relatively constant across a significant concentration range of the NaOH. This region of the titration curve includes which of the following:

A. the half-equivalence point

B. the equivalence point

C. the isoelectric point

D. Planck’s constant

Click for Explanation

The region of the titration curve at which a protic acid pH remains constant includes the half-equivalence point, corresponding to the pKa of the acid, at which the acid is 50% deprotonated. At the pKa, [HA] is 50% and [A] is 50%.

At the equivalence point, the acid is 100% deprotonated, [A]=100%.

The isoelectric point describes the pH at which a molecule carries equal positive and negative charge, usually used to describe amphoteric molecules such as amino acids. Planck’s constant is used in calculations describing quantum phenomenon. Planck’s constant and the isoelectric point have nothing to do with titrations.

A. the half-equivalence point, correct.

B. the equivalence point, incorrect.

C. the isoelectric point, incorrect.

D. Planck’s constant, incorrect.

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