[Cooper]

Hi, my textbook defines solute activity as "its concentration corrected for its nonideal behavior at concentrations higher than infinite dilution." It also says that pure water has a value of 1 at pH 7. What does this mean? :s what nonideal behavior is it referring to and why would that change the concentration? When do you ever use activity values?

Thanks,
Cooper

[Borek]

Say you have a weak acid HA. If everything was working ideally, you would determine Ka value once and you would be able to calculate HA/A^-/H⁺ concentration for every solution, not matter what the total concentration of the acid is. Trick is, if you are determining concentration of A^- using spectroscopic method you will find it doesn't exactly follow calculated results, and the difference is a function of a concentration of free ions present in the solution (so called ionic strength of the solution). The simplest way of describing it is to forget about concentration and introduce activity of the substances present in the solution - for ideal, or infinitely diluted solutions, activity of an ion equals its concentration, but the higher the ionic strength, the higher the difference.