pK_a(CH₃CO₂H) = 4.76

Calculate the pH of a solution containing
0.10

CH₃CO₂Na and 0.10

CH₃CO₂H.

K_a =	[H₃O⁺][CH₃CO₂^–]
	[CH₃CO₂H]

[CH₃CO₂^–] ≈ 0.10

[CH₃CO₂H] ≈ 0.10

Relate pK_a (K_a) and pH (H₃O⁺) using

the K_a expression

Extent of reaction low for both solutes
Relate [CH₃CO₂^–] to initial concentration.
Relate [CH₃CO₂H] to initial concentration.

1.74 × 10^–5 =	[H₃O⁺] × 0.10
	0.10

[H₃O⁺] = 1.74 × 10^–5

pH = 4.76

Entering the above into the K_a expression:

When the concentrations of the acid and base of the conjugate pair are equal (irrespective of whether they are both large or both small), at equilibrium

[H₃O⁺] equals K_a.
pH equals pK_a

K_a = [H₃O⁺] ×	[conjugate base]
	[acid]

K_a = [H₃O⁺] ×	[conjugate base]
	[acid]