0.4 Acid-base equilibrium  (Page 2/7)

[link] implies that a 0.1M solution of the acid $HA$ in water should produce $H_3{O}^{+}$ ions in solution with a concentration of 0.1M. In fact, the concentration of $H_3{O}^{+}$ ions, $\left[H_3{O}^{+}\right]$ , can be measured by a variety of techniques. Chemists commonly use ameasure of the $H_3{O}^{+}$ ion concentration called the $\mathrm{pH}$ , defined by:

$$\mathrm{pH}=-\lg \left[H_3{O}^{+}\right]$$

We now observe the concentration $\left[H_3{O}^{+}\right]$ produced by dissolving a variety of acids in solution at a concentration of 0.1M, and the results are tabulated in [link] .

Note that there are several acids listed for which $\left[H_3{O}^{+}\right]=0.1M$ , and $\mathrm{pH}=1$ . This shows that, for these acids, the acid ionization is complete:essentially every acid molecule is ionized in the solution according to [link] . However, there are other acids listed for which $\left[H_3{O}^{+}\right]$ is considerably less than 0.1M and the pH is considerably greater than 1. For each of these acids, therefore, not all of the acidmolecules ionize according to [link] . In fact, it is clear in [link] that in these acids the vast majority of the acid molecules do not ionize, and only a smallpercentage 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 forwhich only a small percentage of molecules ionize. There are seven strong acids listed in [link] . From many observations, it is possible to determine that theseseven acids are the only commonly observed strong acids. 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 betweenstrong acids and weak acids.

Observation 2: percent ionization in weak acids

[link] shows that the pH of 0.1M acid solutions varies from one weak acid to another. If we dissolve 0.1 moles of acid in a 1.0L solution,the fraction of those acid molecules which will ionize varies from weak acid to weak acid. For a few weak acids, using the data in [link] we calculate the percentage of ionized acid molecules in 0.1M acid solutions in [link] .

We might be tempted to conclude from [link] that we can characterize the strength of each acid by the percent ionization of acid moleculesin solution. However, before doing so, we observe the pH of a single acid, nitrous acid, in solution as a function of theconcentration of the acid.

In this case, "concentration of the acid" refers to the number of moles of acid that we dissolvedper liter of water. Our observations are listed in [link] , which gives $\left[H_3{O}^{+}\right]$ , pH, and percent ionization as a function of nitrous acidconcentration.