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 dm3 or 24,000 cm3 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 dm3 at rtp is related to the number of moles:
Calculate the volume of 0.5 mol of carbon dioxide at rtp.
volume = 0.5 × 24 = 12 dm3
Remember that 1 dm3 = 1 000 cm3 so the volume is also 12 000 cm3
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
Calculate the number of moles of hydrogen that occupy 6 dm3 at rtp.
number of moles = 6 ÷ 24 = 0.25 mol