Consider of the following two acids and their ionization constants. Which conjugate base (#HCOO^-# or #CN^-#) is stronger?

Question

#HCOOH#
#K_a = 1.7 x 10^-4#

#HCN#
#K_a = 4.9 x 10^-10#

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Answer 1 · 2017-01-01T17:32:57

The stronger the acid, the weaker the conjugate base..........

Formic acid is clearly a stronger acid than prussic acid. That is the given equilibrium lies farther to the right:

#HX(aq) + H_2O(l)rightleftharpoonsH_3O^+ + X^-#

And #K_a=([H_3O^+][X^-])/([HX(aq)])#

SInce #K_a# for formic acid is five orders of magnitude greater than #HC-=N#, it follows that cyanide ion is a MUCH stronger base than formate.

Given the figures, we could even calculate #K_b# for the following reaction:

#A^(-) + H_2O(l) rightleftharpoons HA(aq) + HO^-#

We know that in water #pK_a+pK_b=14#. And #pK_b# for #N-=C^-# will THUS BE SMALLER than that for #"formate ion"#.