Question

What is the volume of `2` `mol` of oxygen, `O_2`, at STP?

Answer 1

You need to specify (i) a pressure, and (ii) a temperature, before the molar quantity can be calculated.

Explanation:

If we assume standard laboratory conditions, `25` `""^@C` and `1` `atm` (or thereabouts; the standard is slightly different now!), `2L` dioxygen gas represents, `(2L)/(25.4*L*mol^-1)` `""approx 1/12*mol`.

Answer 2

Assuming that the gas is at standard temperature and pressure (STP), one mole of any gas occupies `22.4` `L`. This means the number of moles of `O_2` is `2/22.4=0.089` `mol`.

Explanation:

The Ideal Gas Law makes certain assumptions about the gases: that the particles are very tiny and featureless and that there is no force acting between them. If these assumptions are correct, then one mole of any gas will occupy `22.4` `L` at STP.

The assumptions are not quite the case for `O_2`, but the deviations from it are small enough that we can still use this approach and receive an accurate answer.