What is the chemical activity of a substance?

Question

The question's pretty self-explanatory lol

Answer 1

When you calculate the pH of a strong acid in a dilute solution and then actually measure the pH either with a pH meter or some series of indicators, you are usually exactly on the same answer with either way. For strong acids in concentrated solutions, this is not the case. Calculations will lead to one answer while measurement will lead to a different answer. Why? If you think about it, you reach the conclusion that ions can interfere with each other when they are jammed close together. Make an analogy to the van Der Waals equation for gases. The corrections that have to be made are somewhat explained in the source I attach here. I never had to use calculations like this.