Question

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

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.