# Why is acetic acid called weak acid?

Jun 24, 2017

Because at equilibrium the parent acid remains largely as the parent acid........

#### Explanation:

The strength of an acid, $H X$, is assessed on the basis of the completion of the equilibrium reaction.........

$H X + {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {X}^{-}$

For stronger acids, the equilibrium lies to the RIGHT as written.

For acetic acid, we write the equilibrium as..........

${H}_{3} C - C {O}_{2} H \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {H}_{3} C - C {O}_{2}^{-}$

And for acetic acid, ${K}_{a} = \frac{\left[{H}_{3} {O}^{+}\right] \left[A c {O}^{-}\right]}{\left[H O A c\right]} = {10}^{- 4.76}$....

Given the equilibrium constant, most of the acetic acid is present as $H O A c$. This specifies a weak acid as per the given definition.....