Weak Bases

In-depth explanation of Weak Bases

A weak base accepts a proton from water only partially, establishing the equilibrium B + H2O <-> BH+ + OH-, which is described by the base dissociation constant Kb. Because it produces only a small amount of hydroxide, the solution is only moderately basic. The calculation parallels that for weak acids, but the equilibrium expression is set up for hydroxide concentration, from which pOH and then pH follow. Common weak bases include ammonia, organic amines and the anions of weak acids, which act as bases toward water. Weak bases form the buffer partners of many biological and industrial systems.

Going deeper: Weak Bases

Key ideas: Weak bases accept protons: B + H2O giving BH+ + OH-, with Kb small. Ammonia has Kb 1.8e-5; amines like methylamine are about 4.4e-4; anions of weak acids such as CH3COO- also act as weak bases. Solve with an ICE table just as for weak acids, but the unknown x is [OH-], so finish with pOH then pH.

Common mistakes and connections: The classic slip is reading x as [H+] and getting a pH below 7 for a base. Another is looking up Kb for an anion: it is usually not tabulated, so compute Kb = Kw/Ka of the conjugate acid. Relates to hydrolysis of salts and amine basicity trends in organic chemistry.

Visualization of Weak Bases

Ammonia uses its lone pair on nitrogen to take a proton from water, giving NH4+ and OH-; only a small fraction reacts, so Kb is small. The interactive lesson shows this as a live animation or chart.

Worked example and use case

A 0.10 M ammonia solution with Kb 1.8 x 10^-5 has [OH-] of about 1.3 x 10^-3 M and pH 11.13.

Where Weak Bases fits in Acids, Bases & Electrochemistry

Weak Bases is topic 12 of 105 in Acids, Bases & Electrochemistry.

Related topics

• Strong AcidsBeginner
• Strong BasesBeginner
• Weak AcidsIntermediate
• Acid Dissociation ConstantIntermediate
• Ka and Kb RelationshipIntermediate
• Percent IonizationIntermediate

Browse all 105 topics in Acids, Bases & Electrochemistry