Answer:
The answer is Benzoic acid. Hence, (C) is the correct answer. The acid with the ratio [HA] / [A-] closest to 1 is the best because a buffer is most effective when the amounts of acid and conjugate base are approximately equal and hence their ratio should be close to 1.
Explanation:
pH = - log₁₀[H⁺]
[H⁺] = 10∧-pH
Given,
pH = 4.30
Chloroacetic acid (Ka = 1.35 x 10^-3)
Propanoic acid (Ka = 1.30 x 10^-5)
Benzoic acid (Ka = 6.40 x 10^-5)
Hypochlorous acid (Ka = 3.50 x 10^-8)
Using the Hints,
The required [H+] = 10⁻⁴°³ => 5.011 x 10⁻⁵
Using the formula,
Ka = [H+] [A-] / [HA] and finding [HA] / [A-] values for each of the given acids.
[HA] / [A-] = [H+] /Ka
Substituting the values,
For Chloroacetic acid, [HA] / [A-] = 5.011 x 10⁻⁵ / 1.35 x 10⁻³ => 0.037
For Propanoic acid, [HA] / [A-] = 5.011 x 10⁻⁵ / 1.30 x 10⁻⁵ => 3.85
For Benzoic acid, [HA] / [A-] = 5.011 x 10⁻⁵ / 6.40 x 10⁻⁵ => 0.782
For Hypochlorous acid, [HA] / [A-] = 5.011 x 10⁻⁵ / 3.50 x 10⁻⁸ => 1431.714
As Benzoic acid has [HA] / [A-] value closest to 1, it can make the best buffered solution.
The acid with the
ratio [HA] / [A-] closest to 1 is the best because,
A buffer is most effective when the amounts of acid and conjugate base are approximately equal and hence their ratio should be close to 1.
