Question

Consider the following weak acids and their Ka values: Acetic Acid Ka = 1.8×10^−5 Phosphoric Acid Ka = 7.5×10^−3 Hypochlorous Acid Ka = 3.5×10^−8. What weak acid-conjugate base buffer system from the acids listed is the best choice to prepare the following buffers. Explain your reasoning.
a. pH 2.8
b. pH 4.5
c. pH7.5

Answer 1

Answer:

a. Phosphoric Acid

b. Acetic Acid

c. Hypochlorous Acid

Explanation:

A buffer works when the pH of this one is in pKa ± 1. That means, to find which buffer system works in some pH you need to find pKa:

pKa = -log Ka

pKa Acetic acid:

-log1.8x10⁻⁵ = 4.74

pKa phosphoric acid:

-log7.5x10⁻³ = 2.12

pKa hypochlorous acid:

-log3.5x10⁻⁸ = 7.46

a. For a pH of 2.8 the best choice is phophoric acid because its effective range is: 1.12 - 3.12 and 2.8 is between these values.

b. pH 4.5. Acetic acid. effective between pH's 3.74 - 5.74

c. pH 7.5. Hypochlorous acid that works between 6.46 and 8.46