1. What's the difference between activity and concentration?

2. At what condition they become same?

3. Why do we need activity at all?

4. Why can't we simply use concentration?

1. What’s the difference between activity and concentration?

Activity is an effective concentration wher ion-ion and ion-H2O interactions are accounted for. Concentration is related to activity by: a = yc

2. At what condition they become same?

They can be equal when y=1 especially when dealing with dilute aqueous solutions.

3. Why do we need activity at all?

Activity is important because it accounts all of the interactions available.

4. Why can’t we simply use concentration?

This is because concentration is an idealized expression it does not account of all the interactions present.