The following is a list of acids
and their acid dissociation constants. (i) Nitrous acid, HNO2, 𝐾 𝑎
equals 4.1 times 10 to the negative fourth moles per liter. (ii) Hypochlorous acid, HClO, 𝐾 𝑎
equals 3.0 times 10 to the negative eighth moles per liter. (iii) Ethanoic acid, CH3COOH, 𝐾 𝑎
equals 1.8 times 10 to the negative fifth moles per liter. (iv) Methanoic acid, HCOOH, 𝐾 𝑎
equals 1.8 times 10 to the negative fourth moles per liter. What is the strongest acid?
To answer this question, we need to
use the provided acid dissociation constants to determine which of the four acids is
An acid dissociation constant is
the equilibrium constant for the reaction of an acid with water. It can be used as a quantitative
measurement of how strong a particular acid is. In its most basic form, we can
state that the acid dissociation constant is equal to the concentration of the
products divided by the concentration of the reactants. Let’s see how this equation can be
applied to the dissociation of a generic weak acid in water.
For this generic reaction equation,
the acid dissociation constant can be expressed as 𝐾 𝑎 equals the concentration of
H+ times the concentration of A− divided by the concentration of HA. If the strength of the acid were to
increase, then more of the acid molecules would dissociate in water and the
equilibrium would shift to the right. So, at equilibrium, a stronger acid
will have a lower HA concentration and higher H+ and A− concentrations than a weaker
acid. This means that the numerator of
the 𝐾 𝑎 equation will be larger, while the denominator will be smaller, resulting
in a larger 𝐾 𝑎 value.
From this, we can see that as acid
strength increases, so too will the acid dissociation constant. So, to determine which acid is the
strongest, we need to identify which acid has the largest 𝐾 𝑎 value. Of the acids provided, the acid
with the largest acid dissociation constant is nitrous acid. Therefore, the strongest acid is
nitrous acid, HNO2.