Conjugate Acids and Conjugate Bases For Various Compounds

Conjugate acid	Formula	Structure	pKa	Conjugate base	Name
Methane	CH4		50-60		Methyl
Ethene	CH2CH2		45		Ethenyl
Ammonia	NH3		36		Amide
Acetylene (ethyne)	C2H2		26		Acetylide
Ethanol	C2H5OH		16		Ethoxide
Water	HOH		15.7		Hydroxide
Ammonium	NH4+	H—NH3	9.8	: NH3	Ammonia
Hydroflouric acid	HF		4		Flouride

Use this table to predict which conjugate base will favorably react with which conjugate acids. Pay attention to the pK_a values shown. As a general rule, the conjugate base of any acid will react with, and remove, the proton (H⁺ ion) from any conjugate acid that is stronger than the conjugate acid from which the conjugate base you are looking at was derived from. 

In the chart above, the conjugate base will remove a proton from every conjugate acid that is below it in the table.

Examples:

• The Amide ion will remove (quantitatively) a proton from Acetylene, Ethanol, Water, and Hydroflouric acid, these cells are shown in color above, so that the amide ion will remove a proton from the acids below it in this table (smaller pK_a values, stronger acids).
• The Hydroxide ion will remove (quantitatively) a proton only from Hydroflouric acid and the Ammonium ion.
• The Ethenyl ion will remove (quantitatively) a proton from Ammonia, Acetylene, Ethanol, Water, and Hydroflouric acid.
• The Methyl carbanion will remove (quantitatively) a proton from all acids (from Ethene, Ammonia, Acetylene, Ethanol, Water, and Hydroflouric Acid)

When I write — quantitatively — I mean that there is enough difference in the pK_a values (of the conjugate acids) to remove greater than 99% of the protons from the acids below them. A difference of 2 pK_a values will allow the conjugate base coming from the weaker acid will remove >99% of protons from the stronger acid.