Understanding the strength of acids is crucial in chemistry, particularly when comparing substances like acetic acid and ammonium. The dissociation constant (K_a) is a key factor in determining acid strength, and it can be transformed into pK_a using the formula:

pK_a = -log(K_a)

For instance, if the dissociation constant yields a value that, when the negative logarithm is taken, results in approximately 9.24, this value represents the pK_a of the acid. A lower pK_a indicates a stronger acid. In this case, acetic acid is identified as the stronger acid compared to ammonium because it has a lower pK_a.

It's important to note that a decrease of 1 unit in pK_a corresponds to a tenfold increase in acid strength, reflecting the logarithmic nature of the scale. Therefore, acetic acid is significantly stronger than ammonium, with estimates suggesting it could be nearly 100,000 times more effective as an acid.

In summary, the trend to remember is that the strongest acids have the lowest pK_a values, while the weakest acids have the highest pK_a values. This concept is fundamental in acid-base chemistry, emphasizing that pK_a is exclusively related to acids and should not be confused with bases.