# Deviation from ideal solutions - activities and concentrations

What are some examples of solutions deviating significantly from ideal behavior, and, by extension, showing pHs significantly different from those predicted by usage of concentrations instead of activity coefficients in K expressions?

I know Wikipedia lists two examples but what other examples are there? Do all dilute solutions behave ideally or mostly ideally? Or are there major exceptions in which the use of concentrations instead of activities fails?

Also, is there an easy way to incorporate activities into the calculation of pH for complex systems?

• To partly answer your last question, we can replace concentrations with activities in just about any expression, especially in equilibrium calculations. For example $K_a=\dfrac{\alpha_{\ce{A-}} \cdot \alpha_{\ce{H+}}}{\alpha_{\ce{HA}}}$ and $\text{pH}=-\log_{10}\alpha_{\ce{H+}}$. Of course, the harder part is usually determining the activities. – Ben Norris Jul 19 '14 at 19:24