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

Question

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

a)

Ka of formic acid = 1.69 x 10^-4

Ka of ammonium (NH4+) = 5.62 x 10^-10

which one has greater Ka value ,that is stroner acid . here formic acid Ka value greater than NH4+ . so

formic acid (HCOOH) is stronger acid

b)

conjugate base of HCOOH = HCOO- (formate ion)

conjugate base of NH4+ = NH3 (ammonia)

Kb = Kw / Ka

Kb of HCOO- = 5.92 x 10^-11

Kb of NH3 = 1.78 x 10^-5

the conjugate base coming from weak acid , that is stronger base .

which base is coming from strong acid , that is weak conjugate base. Kb is NH3 is greater than Kb of HCOO-

here NH3 is strong conjugate base.

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