Question

Benzoic acid is a weak monoprotic acid. It has a pKa of 4.20. A student transferred 25.00mL of 0.126M benzoic acid into a beaker. The student then added 20.00mL of distilled water into the beaker. Calculate the equilibrium pH for this benzoic acid solution.

Answer 1

pH of the benzoic acid solution = 3.35

Explanation:

Benzoic acid is a weak monoprotic acid

pKa = 4.2

⇒ Ka = 6.3 x 10⁻⁵

C₆H₅COOH ⇄ C₆H₅COO⁻ + H⁺

Initial concentration C molar 0 0

At equilibrium (c - cα) cα cα

Acid dissociation constant Ka =
`(c\alpha X c\alpha )/(c - c\alpha )`

⇒ 6.3 x 10⁻⁵ = α²c ................(1)

(for weak electrolyte 1 -α = 1)

Concentration of benzoic acid C = 25 x 0.126 x 10⁻³ = 0.00315 molar

From eqn (1)

α² =
`(6.3 X 10^(-5) )/(0.00315)`

⇒ α = √(
`(6.3 X 10^(-5) )/(0.00315)` )

⇒ α = 0.14

So concentration of H⁺ ion = 0.14 x 0.00315 = 0.000441

pH = - log 0.000441 = 3.35

We not considered H⁺ concentration of water because pH < 7