Question

K_a values for acids H₂SO₃, HNO₂, CH₃COOH and HCN are respectively 1.3 × 10^-2, 4 × 10^-4, 1.8 × 10^-5 and 4 × 10^-10, which of the above acids produces stronger conjugate base in aqueous solution ?

• A. H₂SO₃
• B. HNO₃
• C. CH₃COOH
• D. HCN

Answer 1

The Correct Option is D

1. Understanding the Relationship Between Kₐ and Conjugate Base Strength:
The strength of a conjugate base is inversely related to the strength of its parent acid. A weaker acid (smaller Kₐ) produces a stronger conjugate base. The Kₐ value represents the acid dissociation constant, and a smaller Kₐ indicates less dissociation, meaning the acid is weaker and its conjugate base is stronger.

2. Listing the Acids and Their Kₐ Values:
We are given:
- H₂SO₃: Kₐ = 1.3 × 10⁻²
- HNO₂: Kₐ = 4 × 10⁻⁴
- CH₃COOH: Kₐ = 1.8 × 10⁻⁵
- HCN: Kₐ = 4 × 10⁻¹⁰

3. Comparing the Kₐ Values:
To find the weakest acid, we compare the Kₐ values. The smaller the Kₐ, the weaker the acid, and thus the stronger its conjugate base:
- H₂SO₃: 1.3 × 10⁻² (largest Kₐ, strongest acid)
- HNO₂: 4 × 10⁻⁴
- CH₃COOH: 1.8 × 10⁻⁵
- HCN: 4 × 10⁻¹⁰ (smallest Kₐ, weakest acid)

4. Identifying the Weakest Acid:
HCN has the smallest Kₐ value (4 × 10⁻¹⁰), making it the weakest acid among the given options.

5. Determining the Strongest Conjugate Base:
The conjugate base of HCN is CN⁻. Since HCN is the weakest acid, CN⁻ is the strongest conjugate base in aqueous solution.

Final Answer:
The acid that produces the strongest conjugate base is HCN, so the answer is (D) HCN.

Answer 2

The strength of an acid is inversely related to the strength of its conjugate base. The stronger the acid (higher \( K_a \)), the weaker the conjugate base. Conversely, the weaker the acid (lower \( K_a \)), the stronger the conjugate base. Since HCN has the lowest \( K_a \) value, it will produce the strongest conjugate base in aqueous solution.

The correct answer is (D) : HCN.