Question

how would increasing the original concentration of a weak acid affect its ka and pka?

A.
Ka and pKa would increase.
B.
Ka and pKa would decrease.
C.
Ka would increase and pKa would decrease.
D.
Ka would decrease and pKa would increase.
E.
Ka and pKa would both stay the same.

Answer 1

Answer: C. $K_a$ would increase and $pK_a$ would decrease.

Explanation:

$HA\rightleftharpoons H^++A^-$

initially conc.       c       0         0

At eqm.  $c(1-\alpha)$   $c\alpha$      $c\alpha$

The expression for dissociation constant is,

$K_a=\frac{c\alpha\times c\alpha}{c(1-\alpha)}$

for weak acid, $\alpha$ is very very small , the expression will be,

$K_a=\frac{c^2\alpha^2}{c}=c\alpha^2$

$K_a=c\alpha^2$As $k_a$ is directly proportional to concentration, the value of $K_a$ would increase on increasing the concentration.

$pK_a=log\frac{1}{K_a}$

Thus if $K_a$ would increase, $pk_a$ would decrease.