what the difference between the effective concentration of hydrogen ions (or the activity) and the actual concentration?.
In chemistry, pH is a measure of the activity of the (solvated) hydrogenion H+, which measures the hydrogen ion concentration is closely related to,and is often written as, pH. Activity can be thought of as applying a correction factor to the concentration. The activity coefficient of a solute is constant in any particular given solution but its value can change if the properties of the solution are changed (eg by changing the ionic strength or the temperature). Actual concentration, in chemistry, is a measure of the relative proportions of two or more quantities in a mixture. Concentration may be expressed in a number of ways. The simplest statement of the concentrations of the components of a mixture is in terms of their percentages by weight or volume.