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Sample MCAT question: Acid base pairs

  • This MCAT practice question is supplied courtesy of Next Step Test Preparation. Next Step is national leader in MCAT test prep and focuses exclusively on providing one-on-one, personalized test prep for the MCAT.

Question: Which of the following acid/base pairs yields the most nearly neutral aqueous solution when mixed?

a) HCl/NaF

 b) H2CO3/2 NaOH

c) HCl/NaOH

d) HBr/NH3

Explanation:

a) incorrect, strong acid + weak base forms NaCl and HF, the HF hydrolyzes to H3O+ and F-, pH<7

b) incorrect, weak acid + strong base forms Na2CO3 + 2 H2O, and the CO32- ion autoionizes water, to create OH-  ions, pH>7

c) correct, strong acid + strong base forms NaCl + H2O and salt ions do not react with water to change pH, pH=7

d) incorrect, strong acid + weak base forms NH4Br, the NH4+ ions weakly dissociates to NH3 + H+ and Br- does not react with water, pH<7

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