What is the equilibrium constant for the following acid–base reactions?(a) ammonia and ace...

What is the equilibrium constant for the following acid–base reactions?

(a) ammonia and acetic acid

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(b) fluoride ion and acetic acid

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(c) ethanol and hydrobromic acid

Sample Solution (a) Always start with an equation for an acid-base reaction. Ammonia is a Brønsted base and accepts a proton from the —OH group of acetic acid. Ammonia is converted to its conjugate acid, and acetic acid to its conjugate base.

From their respective pK_a’s, we see that acetic acid is a much stronger acid than ammonium ion. Therefore, the equilibrium lies to the right. The equilibrium constant for the process is

An unexpected fact emerges by working through this exercise. We see that although acetic acid is a weak acid and ammonia is a weak base, the acid-base reaction between them is virtually complete.