Video Transcript
Ethanoic acid is a weak acid that
dissociates according to the following: CH3COOH is in equilibrium with H+ plus
CH3COO−. What is the acid dissociation
constant for this acid?
An acid dissociation constant is
the equilibrium constant for the reaction of an acid with water. Acid dissociation constants can be
used to provide a quantitative measurement of the strength of an acid. The higher the value of 𝐾 𝑎, the
stronger the acid. The acid dissociation constant can
be calculated by dividing the concentration of the products by the concentration of
the reactants. Let’s see how this can be applied
to a generic weak acid represented by HA.
We can simplify the reaction of a
weak acid with water by just showing the disassociation of the acid into H+ and A−
ions. This means that the acid
dissociation constant for a generic weak acid would be 𝐾 𝑎 equals the
concentration of H+ times the concentration of A− divided by the concentration of
HA.
We can apply this understanding to
the dissociation of ethanoic acid to write an acid dissociation constant
expression. We can start by writing the
concentration of each of the products in the numerator. We can complete the expression by
writing the concentration of ethanoic acid in the denominator. This gives us our final
equation. The acid dissociation constant for
ethanoic acid is 𝐾 𝑎 equals the concentration of H+ times the concentration of
CH3COO− divided by the concentration of CH3COOH.
