1.20 understand the term molar volume of a gas and use its values (24 dm3 and 24,000 cm3) at room temperature and pressure (rtp) in calculations.

At room temperature and pressure, one mole of any gas occupies 24 dm3 (= 24,000 cm3).

That makes life pretty easy. If we know the volume of a gas, we know how many moles of that gas we have, and vice-versa.

MOLES = VOLUME ÷ MOLAR VOLUME VOLUME = MOLES × MOLAR VOLUME

(when you use these equations, pick the right units, so use 24,000 if you want are dealing with a volume in cm3 and 24 if dealing with cubic decimetres (dm3).  (Note: 1 dm³ is the same thing as a litre.)

e.g. 1    what volume would 0.02 mol of hydrogen gas occupy?

Volume = moles × molar volume

= 0.02 x 24 dm3

= 0.48 dm3

e.g. 2    how many moles of chlorine are there in 386 cm3 of chlorine gas?

Moles   = volume ÷ molar volume

= 386 cm3 ÷ 24000

= 0.0161 mol