Question

How many molecules of neon are associated with a `75*L` volume of neon gas?

Answer 1

The question should have specified a temperature. I am going to assume that the gas is at room temperature, `298*K`.

Explanation:

We know that at room temperature and pressure, `1` `mol` of ideal gas occupies a volume of `25.4*L`.

We assume (reasonably) that `Ne` behaves ideally.

Thus the number of moles of `Ne` `=` `(75*L)/(25.4*L*mol^-1)` `~=` `3*mol`.

But by definition, `1*mol` of stuff possesses `6.022xx10^23` individual items of that stuff. If you can grasp this concept, it will save you a ton of angst.

Thus `"Molecules (atoms) of neon"` `=` `(75*L)/(25.4*L*mol^-1)xx6.022xx10^23" neon atoms per mole"`

`~=18xx10^23 " neon atoms"`.

What is the mass of this quantity of neon gas?

(Neon gas is a monatomic molecule).

See also here.