Which of the following acids has the weakest conjugate base in aqueous solution?
A.CH3COOH
B.HF
C.HOCl
D.HNO2
E.Hcl
The Correct Answer and Explanation is:
The correct answer is: E. HCl
Explanation
To determine which acid has the weakest conjugate base, we must first understand the relationship between acid strength and conjugate base strength.
According to Brønsted-Lowry theory:
- A strong acid donates protons easily and dissociates completely in water.
- Its conjugate base has little tendency to accept a proton, making it very weak.
- A weak acid, in contrast, only partially dissociates in water, and its conjugate base is relatively stronger.
Thus, the stronger the acid, the weaker its conjugate base.
Now let’s examine each option:
- A. CH₃COOH (acetic acid): A weak organic acid. Its conjugate base, CH₃COO⁻, is moderately strong.
- B. HF (hydrofluoric acid): Though it contains a halogen, HF is not a strong acid due to strong hydrogen bonding. Its conjugate base F⁻ is relatively stronger.
- C. HOCl (hypochlorous acid): A weak acid. Its conjugate base OCl⁻ is relatively stronger.
- D. HNO₂ (nitrous acid): Also a weak acid. The conjugate base NO₂⁻ is stronger than that of a strong acid.
- E. HCl (hydrochloric acid): A strong acid that dissociates completely in water:
HCl → H⁺ + Cl⁻
Its conjugate base Cl⁻ is extremely weak because it has negligible tendency to re-accept a proton.
Conclusion:
Among the listed acids, HCl is the strongest. Therefore, its conjugate base (Cl⁻) is the weakest. This makes option E the correct answer. This question tests understanding of acid-base equilibrium and the inverse relationship between acid strength and conjugate base strength — a core concept in general and analytical chemistry.
