Reaction (1) implies that a 0.1 M solution of the acid HA in water should produce H 3 O + ions in solution with a concentration of 0.1 M. In fact, the concentration of H 3 O + ions, [H 3 O + ], can be measured by a variety of techniques. Chemists commonly use a measure of the H 3 O + ion concentration called the pH, defined by:
pH = -log 10 [H 3 O + ]
We now observe the concentration [H 3 O + ] produced by dissolving a variety of acids in solution at a concentration of 0.1 M, and the results are tabulated in Table 1.
| H 2 SO 4 | 0.1 | 1 |
| HNO 3 | 0.1 | 1 |
| HCl | 0.1 | 1 |
| HBr | 0.1 | 1 |
| HI | 0.1 | 1 |
| HClO 4 | 0.1 | 1 |
| HClO 3 | 0.1 | 1 |
| HNO 2 | 6.2∙10 -3 | 2.2 |
| HCN | 7∙10 -6 | 5.1 |
| HIO | 1∙10 -6 | 5.8 |
| HF | 5.5∙10 -3 | 2.3 |
| HOCN | 5.5∙10 -3 | 2.3 |
| HClO 2 | 2.8∙10 -2 | 1.6 |
| CH 3 COOH (acetic acid) | 1.3∙10 -3 | 2.9 |
| CH3CH 2 COOH (propionic acid) | 1.1∙10 -3 | 2.9 |
Note that there are several acids listed for which [H 3 O + ] = 0.1 M, and pH = 1. This shows that, for these acids, the acid ionization is complete: essentially every acid molecule is ionized in the solution according to Reaction (1). However, there are other acids listed for which [H 3 O + ] is considerably less than 0.1 M and the pH is considerably greater than 1. For each of these acids, therefore, not all of the acid molecules ionize according to Reaction (1). In fact, it is clear in Table 1 that in these acids the vast majority of the acid molecules do not ionize, and only a small percentage does ionize.
From these observations, we distinguish two classes of acids: "strong acids" and "weak acids." Strong acids are those for which nearly 100% of the acid molecules ionize, whereas weak acids are those for which only a small percentage of molecules ionize. There are seven strong acids listed in Table 1. From many observations, it is possible to determine that these seven acids are the only commonly observed strong acids. Otherwise, the vast majority of all substances with acidic properties are weak acids. We seek to characterize weak acid ionization quantitatively and to determine what the differences in molecular properties are between strong acids and weak acids.
Observation 2: percent ionization in weak acids
Table 1 shows that the pH of 0.1 M acid solutions varies from one weak acid to another. If we dissolve 0.1 moles of acid in a 1.0 L solution, the fraction of those acid molecules that will ionize varies from weak acid to weak acid. For a few weak acids, using the data in Table 1 we can calculate the percentage of ionized acid molecules in 0.1 M acid solutions in Table 2.
| HNO 2 | 6.2∙10 -3 | 6.2% |
| HCN | 7∙10 -6 | 0.007% |
| HIO | 1∙10 -6 | 0.001% |
| HF | 5.5∙10 -3 | 5.5% |
| HOCN | 5.5∙10 -3 | 5.5% |
| HClO 2 | 2.8∙10 -2 | 28.2% |
| CH 3 COOH (acetic acid) | 1.3∙10 -3 | 1.3% |
| CH3CH 2 COOH (propionic acid) | 1.1∙10 -3 | 1.1% |
We might be tempted to conclude from Table 2 that we can characterize the strength of each acid by the percent ionization of acid molecules in solution. However, before doing so, we observe the pH of a single acid, nitrous acid (HNO 2 ), in solution as a function of the concentration of the acid.
HNO 2 (aq) + H 2 O(l) → H 3 O + (aq) + NO 2 - (aq)
In this case, the "concentration of the acid" refers to the number of moles of acid that we dissolved per liter of water. Our observations are listed in Table 3, which gives [H 3 O + ], pH, and percent ionization as a function of nitrous acid concentration.