Overview with all three cases

Irrespective of the initial concentrations of the acid and its conjugate base, [acid], [conjugate base] and [H₃O⁺] combine according to the K_a expression to give the same number. This number is a characteristic of the conjugate pair.
 

Ka =	[CH3CH2CO2–][H3O+]
	[CH3CH2CO2H]

	initial concentrations
	c(CH3CH2CO2–)	c(CH3CH2CO2H)	pH
Solution 1	0	0.100	2.94
Solution 2	0.100	0	8.94
Solution 3	0.100	0.100	4.89

	equilibrium concentrations from previous pages
	[CH3CH2CO2–]	[CH3CH2CO2H]	[H3O+]	Ka
Solution 1	1.15×10–3	0.099	1.15 ×10–3	1.3 ×10–5
Solution 2	0.100	8.77×10–6	1.15 ×10–9	1.3 ×10–5
Solution 3	0.100	0.100	1.29 ×10–5	1.3 ×10–5

Note that

• The concentration of substances dissolved (in bold italics) is substantially unchanged at equilibrium. This is because the reaction to reach equilibrium occurs to a small extent.
   
• In solutions 1 and 2, the only source of the conjugate of the substance dissolved is the reaction to reach equilibrium.
   
• in solution 1 
  the reaction to reach equilibrium gives equal amounts of the conjugate base and H₃O⁺.  At equilibrium these concentrations are equal.
  CH₃CH₂CH₂CO₂H + H₂O  CH₃CH₂CO₂^– + H₃O⁺
   
• In solution 2 
  the reaction to reach equilibrium gives equal amounts of the conjugate acid and OH^–.  At equilibrium these concentrations are equal to K_w/[H₃O⁺].
  CH₃CH₂CH₂CO₂^– + H₂O  CH₃CH₂CO₂H + OH^–