Calculating Percent Dissociation



 

Learning Objective

  • Calculate percent dissociation for weak acids from their Ka values and a given concentration.

Key Points

    • 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.

Terms

  • 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.

Example

Calculate the percent dissociation of a weak acid in a [latex]0.060\;M[/latex] solution of HA ([latex]K_a=1.5\times 10^{-5}[/latex]).

To determine percent dissociation, we first need to solve for the concentration of H+. We set up our equation as follows:

[latex]K_a=\frac{[H^+][A^-]}{[HA]}[/latex]

[latex]1.5\times 10^{-5}=\frac{x^2}{0.060-x}[/latex]

However, because the acid dissociates only to a very slight extent, we can assume x is small. The above equation simplifies to the following:

[latex]1.5\times 10^{-5}\approx \frac{x^2}{0.060}[/latex]

[latex]x=[H^+]=9.4\times 10^{-4}[/latex]

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%:

[latex]\alpha = \frac{[H^+]}{[HA]}\times 100\%=\frac{9.4\times 10^{-4}}{0.060}\times 100\%=1.6\%[/latex]

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.

Ka from Percent DissociationIn this video, the percent dissociation of the acid is significant (11%), and se we cannot assume x is small. An ICE chart is used to calculate the relevant concentrations.