Can someone give me a general definition of activity and what it is used for? I am only in general chem . . .

Answer 1

OK, this is an oversimplified explanation so it is not 100% accurate.

In real soltuions there are several factors which make the behaviour deviate from the ideal. E.g. when you have ions you have ion-ion interactions. Thus the effective concentration which determines the chemical reactivity is different than the "absolute" concentration. Activity is this effective concentration and it is
α =γ* C where α activity, γ the activity coefficient and C the Molarity (in physical chemistry molality is prefered over Molarity so α =γ *m). So for example equilibrium constant expressions are usually expressed based on activities rather than concentrations.

For example we say that pH=-log[H+]. However the pH-meter actually measures the effective concentration of H+, that is the activity of H+, αH+ . So pH= -logαH+
For very dilute solutions γ~1 so αH+ ~[H+]
For moderately dilute solutions 0<γ<1 so αH+ <[H+]
For very concentrated solutions, things become complicated so I won't get into that.