Question

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

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

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

Answer 1

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

Explanation:

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"`.