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Given that a base has a Kb value of 8.36×10−4, what is the Ka of its conjugate acid?
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17. Acid and Base Equilibrium
Ka and Kb
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Topic: Ka and Kb. Which of the following is the strongest base in water?
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Identify the species given: OH⁻ (hydroxide ion), F⁻ (fluoride ion), Br⁻ (bromide ion), and Cl⁻ (chloride ion). These are all common bases in water.
Recall that the strength of a base in water is related to its ability to accept a proton (H⁺), which is inversely related to the strength of its conjugate acid. The stronger the conjugate acid, the weaker the base, and vice versa.
Look up or recall the acid dissociation constants (Ka) of the conjugate acids: water (H₂O) for OH⁻, HF for F⁻, HBr for Br⁻, and HCl for Cl⁻. Strong acids have very small Ka values (or very large Ka values depending on definition), but more importantly, their conjugate bases are weaker.
Compare the relative acid strengths: HCl and HBr are strong acids, so Cl⁻ and Br⁻ are very weak bases. HF is a weak acid, so F⁻ is a stronger base than Cl⁻ and Br⁻. Water is a very weak acid, so OH⁻ is the conjugate base of a very weak acid, making it the strongest base among the four.
Conclude that OH⁻ is the strongest base in water because it is the conjugate base of water, which is a very weak acid, and thus OH⁻ has the greatest tendency to accept a proton compared to F⁻, Br⁻, and Cl⁻.
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