In class we discussed how the strength of an acid is inversely proportional to t

Question

In class we discussed how the strength of an acid is inversely proportional to the strength of its conjugate base (Think HCl is very strong while Cl- is so weak you wouldn't even consider it a base). Now, let's consider two weak acids discussed in class: formic acid (CHOOH) Ka = 1.69 x 10-4 and ammonium (NH4+) Ka = 5.62 x 10-10.

a) Identify which of these two is the stronger acid, and justify your answer based on what the Ka represents

b) Show mathematically that the initial statement is true by comparing the relative strengths of the acids above and strengths of their conjugate bases, formate and ammonia, respectively, using Kb and it's meaning to justify your answer.

Explanation / Answer

Ans. #a. Strength of an acid is proportional to its Ka value – greater is the Ka value, stronger is the acid.

# Formic acid is the stronger acid because its Ka value is greater than that of NH4+.

#b. Kb of formate ion (conjugate base of formic acid) =

10-14 / Ka = 10-14 / (1.69 x 10-4) = 5.92 x 10-11

# Kb of ammonia (conjugate base of ammonium ion) =

10-14 / Ka = 10-14 / (1.69 x 10-4) = 1.78x 10-5

Strength of an acid is inversely proportional to strength (Kb) of its conjugate base.

That’s is, smaller is the Kb value of a conjugate base, stronger is the acid.

# Therefore, Formic acid is the stronger acid because Kb value of its conjugate base is smaller than that of the conjugate base of NH4+.

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