In chemistry, what varies with the number of molecules present in a sample of a particular substance?

concentration

Explanation:

Concentration refers to the amount of a substance present in a sample. The more molecules of a substance present in a sample, the greater its concentration. The less molecules of a substance in a sample, the lesser the concentration. We are often concerned about analytically determining the concentration of a substance using diverse analytical methods in chemistry.

the answer is the

it should be p1 v1 = p2 v2