Calculate percent dissociation for weak acids from their Ka values and a given concentration.
Percent dissociation is symbolized as α (alpha) and represents the ratio of the concentration of dissociated hydrogen ion [H+] to the concentration of the undissociated species [HA].
Unlike Ka, percent dissociation varies with the concentration of HA; dilute acids dissociate more than concentrated ones.
Percent dissociation is related to the concentration of both the conjugate base and the acid's initial concentration; it can be calculated if the pH of the solution and the pKa of the acid are known.
percent ionizationthe fraction of an acid that undergoes dissociation
dissociationthe process by which compounds split into smaller constituent molecules, usually reversibly.
We have already discussed quantifying the strength of a weak acid by relating it to its acid equilibrium constant Ka; now we will do so in terms of the acid's percent dissociation. Percent dissociation is symbolized by the Greek letter alpha, α, and it can range from 0%< α < 100%. Strong acids have a value of α that is equal to or nearly 100%; for weak acids, however, α can vary, depending on the acid's strength.
Calculate the percent dissociation of a weak acid in a
solution of HA (
To determine percent dissociation, we first need to solve for the concentration of H+. We set up our equation as follows:
However, because the acid dissociates only to a very slight extent, we can assume x is small. The above equation simplifies to the following:
To find the percent dissociation, we divide the hydrogen ion's concentration of by the concentration of the undissociated species, HA, and multiply by 100%:
As we would expect for a weak acid, the percent dissociation is quite small. However, for some weak acids, the percent dissociation can be higher—upwards of 10% or more. For example, with a problem involving the percent dissociation of a 0.100 M chloroacetic acid, we cannot assume x is small, and therefore use an ICE table to solve the problem.
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