MCAT General Chemistry Question 282: Answer and Explanation

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Question: 282

1. A local factory accidentally pollutes groundwater supplies near a cave with (NH4)2CO3. How would the cave most likely be affected?

  • A. Stalagmite and stalactite formation would likely not be affected.
  • B. Increased groundwater acidity would increase stalagmite and stalactite formation.
  • C. Increased groundwater acidity would decrease stalagmite and stalactite formation.
  • D. Cannot be determined from the information given.

Correct Answer: D

Explanation:

D NH4+ is an acidic ion and CO32- is a basic ion. Both could potentially affect the equilibrium described in Equation 3, so choice A seems unlikely. Although ammonium carbonate is a basic salt, eliminating choices B and C, this could not be determined without knowing the relative pKb and pKa values of NH3 and HCO3- respectively. Given the rise in pH of groundwater and that CO32- would likely have a common ion effect on the CaCO3 solubility, less limestone would be expected to dissolve, leading to decreased formations in the cave.

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