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 $"formate ion"$ .