Question

Which solution below would be the best choice for preparing a buffer with a pH = 5.0?

a.

formic acid and sodium formate, K_a = 1.8x10^-4

b.

acetic acid and sodium acetate, K_a = 1.8 x 10^-5

c.

hypochlorous acid and sodium hypochlorite, K_a = 3.5 x10^-8

d.

boric acid and sodium borate, K_a = 5.8 x 10^-10

e.

All of these solutions would be equally good choices for making this buffer.

Answer 1

A buffer system becomes more effective with substances that have a pKa very close to the required pH, that is, because it is necessary to have concentrations of substances and their salts with concentrations closer to each other, this is observed with the Henderson Hasselbach equation:

pH = pKa + Log ([Salt] / [Acid])

With the following equation, we calculate the pKa of the solutions:

pKa = - Log Ka

a) pKa = 3.74

b) pKa = 4.74

c) pKa = 7.46

d) pKa = 9.24

As we can see, option B is the closest to pH = 5, this is the most appropriate option.

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