# What is an expression for the acid ionization constant (K_a) for H_2CO_3?

Jan 9, 2018

${K}_{a} = \frac{\left[{H}_{3} {O}^{+}\right] \left[H C {O}_{3}^{-}\right]}{\left[{H}_{2} C {O}_{3}\right]}$
${H}_{2} C {O}_{3} \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + H C {O}_{3}^{-}$
Note that the existence of carbonic acid in aqueous solution may be a bit fanciful....the species in solution is likely...${H}_{2} O \cdot C {O}_{2} \equiv {H}_{2} C {O}_{3}$..i.e. carbon dioxide solvated by water. You are on your own with finding ${K}_{a}$...it is small.