A hypothetical weak acid, HA, dissociates in water according to the reaction below.
HA + H$_2$O \leftrightarrow A$^-$ + H$_3$O$^+$
If the pH of a 0.500 M aqueous solution of HA is 2.75, what is the value of $K_a$ for HA?
Hint: The simplified equation, HA \leftrightarrow H$^+$ + A$^-$ , for weak acid dissociation could also be used.
6.0 x 10$^{-5}$
4.0 x 10$^{-9}$
1.6 x 10$^{-7}$
6.3 x 10$^{-6}$