Summary more acid than base

When a buffer has a higher concentration of the acid of the conjugate pair than the conjugate base, for the system to be at equilibrium (for K to equal K_a):

[H₃O⁺] must be higher than K_a. because

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

Thus pH must be lower than pK_a.

Making connections to remember this:
For any solution pH is lower if the concentration of the acid is higher.
For a buffer solution pH is lower than pK_a if the concentration of acid is higher.

Provided the concentration of the acid is no more than ten times the concentration of the conjugate base, the pH will be within 1 unit of pK_a.

K_a = [H₃O⁺] ×	1
	10

K_a = [H₃O⁺] × 10^–1

Take negative logarithm of all terms
pK_a = pH + 1
pH = pK_a–1

See the graph which shows pH change on addition of NaOH to CH₃CO₂H (pK_a 4.76). At 2.5

added NaOH the ratio of [conjugate base] to [acid] is 0.1, and the pH is 3.78.

titration curve ethanoic acid with sodium hydroxide	CH₃CO₂^–
	CH₃CO₂H/ CH₃CO₂^–
	CH₃CO₂H

Buffers where the ratio [conjugate base] to [acid] is between 0.1 and 1 have pH between pK_a –1 and pK_a.