Measure of dissociation:

The acidity constant is as well a measure of dissociation and of how acidic a specific acid is. The stronger the acid, the much more it is ionized and the better concentration of products in the above equation. The meaning of this is that a strong acid has a high K_a value. The K_a values for the subsequent ethanoic acids are in brackets and illustrate that the strongest acid in the series is trichloroacetic acid.

CI₃CCO₂H (23200 * 10^-5) > CI₂CHCO₂H (5530 * 10^-5) > CICH₂CO₂H (136*10^-5) > CH₃CO₂H (1.75*10^-5)

K_a values are strange to work with and thus it is more general to measure the acidic strength as a pK_a value than K_a. The pK_a is the negative logarithm of K_a (pK_a = log10 K_a) and results in much more manageable numbers. The pK_a values for every of the above ethanoic acids are displayed in brackets below. The strongest acid (trichloroacetic acid) comprises the lowest pK_a value.

Cl₃CCO₂H (0.63) < Cl₂CHCO₂H (1.26) < ClCH₂CO₂H (2.87) < CH₃CO₂H (4.76).

Hence the stronger the acid, the higher the value of K_a, and the lower the value of pK_a. An amine like ethylamine (CH₃CH₂NH₂) is a very weak acid (pK_a = 40) as compared to ethanol (pK_a = 16). This is because of the relative electronegativities of oxygen and nitrogen as explained above. Though, the electronegativity of neighboring atoms is not just only the effect on acidic strength. For instance, the pK_a values of ethanoic acid (4.76), ethanol (16), and phenol (10) depict that ethanoic acid is very much acidic as compare to the phenol, and that phenol is much more acidic as compared to the ethanol. The variation in acidity is quite marked, yet hydrogen is attached to oxygen in all three structures.

Likewise, the ethanoic acids Cl₃CCO₂H (0.63), ClCH₂CO₂H (2.87), Cl₂CHCO₂H (1.26), and CH₃CO₂H (4.76) have considerably different pK_a values and still the acidic hydrogen is attached to an oxygen in each of these structures. Hence, factors other than electronegativity have a task to play in determining acidic strength.