The relative formula mass of a compound is calculated by adding together the relative atomic mass values for all the atoms in its formula. Moles are units used to measure substance amount.

One mole of any gas has a volume of 24 dm^{3} or 24,000 cm^{3} at **rtp** (room temperature and pressure). This volume is called the **molar volume of a gas**.

This equation shows how the volume of gas in dm^{3} at rtp is related to the number of moles:

volume of gas at rtp = number of moles × 24

- Question
Calculate the volume of 0.5 mol of carbon dioxide at rtp.

volume = 0.5 × 24 = 12 dm

^{3}Remember that 1 dm

^{3}= 1 000 cm^{3}so the volume is also 12 000 cm^{3}The equation can be rearranged to find the number of moles, if the volume of gas at rtp is known:

number of moles = volume of gas at rtp ÷ 24

- Question
Calculate the number of moles of hydrogen that occupy 6 dm

^{3}at rtp.number of moles = 6 ÷ 24 = 0.25 mol