How many molecules in a given volume of fluid?
Well, for liquids you need to be quoted density,
And for gases you need pressure, volume, and temperature......
So let us take an example.....
We know that
And to find the number of molecules we need the molar quantity....
For gases, as well, you know the Ideal Gas equations, and these can be manipulated to give the answer in moles, which represent the number of gaseous particles, of molecules.
And thus here we use
You happy with this? Because this is a fundamental concept to incorporate in your understanding.