MCAT General Chemistry Question 237: Answer and Explanation

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Question: 237

6. The Ka of HSCN is equal to 1 × 10-4. The pH of a HSCN solution:

  • A. will be approximately 4.
  • B. will be approximately 10.
  • C. will increase as [HSCN] increases.
  • D. cannot be determined from the information given.

Correct Answer: D

Explanation:

D The Ka of an acid is a measure of its ability to dissociate in water, not the pH of a solution (the smaller the Ka the weaker the acid). If we know the [H+] of a solution we can find the pH by finding –log [H+], but we cannot find the pH of a weak acid solution from only the Ka. We must also know the concentration of the acid. Choice A is a trap answer if you confuse pKa with pH. The greater the concentration of an acid, the more H+ ions will be in solution. However, this will decrease the pH of the solution, not increase it (choices B and C can be eliminated). By process of elimination, choice D is the best answer.

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