Question

In a solution that contains benzoic acid (pKa = 4.2), 16% of the total benzoic acid occurs as the benzoate anion. What is the pH of this solution?

Consider a solution that contains both acetic and benzoic acid. Using the information above, what is the pH of the solution if 95% of the benzoic acid is dissociated? How much acetic acid is dissociated (e.g., is in the form of acetate) at this pH?

Answer 1

we know that

for buffers

pH = pKa + log [conjugate base / acid]

in this case

pH = pKa + log [benzoate / benzoic acid]

given

16 % is benzoate ion

so 84 % will be benzoic acid

so

pH = 4.2 + log [ 16 / 84]

pH = 3.48

so

the pH of this solution is 3.48

2)

now

given

95% of the benzoic acid is dissociated

so

benzoic acid = 5

benzoate = 95

so

pH = pKa + log [benzoate / benzoic acid]

pH = 4.2 + log [95 / 5]

pH = 5.47875

now

we know that

for acetic acid pKa is 4.76

so

5.47875 = 4.76 + log [acetate / acetic acid]

[acetate / acetic acid] = 5.233

[acetate] = 5.233 [acetic acid]

now

[acetic acid] + [acetate] = 100

[acetic acid] + 5.233 [acetic acid] = 100

6.233 [acetic acid] = 100

[acetic acid] = 16.04

[acetate] = 100 - 16.04 = 83.96

so

83.96 % of the acetic acid is dissociated