MCAT General Chemistry Question 231: Answer and Explanation
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Question: 231
9. The Ksp of NaCl in water is 35.9 at 25°C. If 500 mL of 12 M NaOH(aq) and 500 mL of 12 M HCl(aq) solution both at 25°C are combined, what would best describe the resulting solution?
- A. A small amount of NaCl(s) would precipitate.
- B. There will be a 6 M aqueous solution of NaCl.
- C. Enthalpy and entropy would increase.
- D. The resulting solution would be slightly basic.
Correct Answer: B
Explanation:
B Neutralizations are exothermic and form salt and water. The starting 500 mL solutions contain 6 moles each of NaOH and HCl. The final solution will be 1 L of 6 M NaCl(aq):
6 HCl(aq) + 6 NaOH(aq) → 6 NaCl(aq) + 6 H2O(l)
Although the reaction quotient of NaCl in this resulting solution will slightly exceed Ksp:
Q = [Na+][Cl-] = [6][6] = 36
the temperature will be significantly increased, allowing more NaCl to dissolve (Ksp will increase with temperature), eliminating choice A. Choice C is eliminated because enthalpy significantly decreases as heat is given off in this exothermic reaction. Choice D is eliminated because NaCl is a neutral salt.