Question

What is ${\mathrm{K}}_{\mathrm{a}}$ in chemistry?

Answer 1

Acid dissociation constant:

• The equilibrium constant of the dissociation of acid in water is known as the acid dissociation constant.
• It is denoted by ${\mathrm{K}}_{\mathrm{a}}$.
• Consider the dissociation of Hydrochloric acid $\left(\mathrm{HCl}\right)$ which dissociates to give ${\mathrm{A}}^{–}$and Hydronium ion$\left({\mathrm{H}}_3{\mathrm{O}}^{+}\right)$.
• The chemical equilibrium can be represented as:

$\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\rightleftharpoons {\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right)$

Expression for acid dissociation constant:

• The dissociation constant can be expressed as:

$K_{\mathrm{a}}=\frac{\left[{\mathrm{Cl}}^{-}\right]\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]}{\left[\mathrm{HCl}\right]\left[{\mathrm{H}}_2\mathrm{O}\right]}$