大象传媒

Gas volume calculations - higher tier

1 mol of any gas at 20掳C and 1 atm pressure occupies a volume of 24 dm3 (which is the same as 24 000 cm3). This volume is known as the molar gas volume (Vm).

The volume of a gas may be calculated from its number of moles using:

volume of gas = moles 脳 Vm

rearranging this:

\(moles~of~gas = \frac {volume}{V_m} \)

The volume of gas and the Vm are always in the same units. If you are using a gas volume in cm3, then Vm = 24 000 cm3. Similarly if the Vm you use is 24 dm3 then the gas volume calculated will be in dm3.

Example: Calculate the volume of hydrogen gas produced in cm3 when 0.3 g of magnesium ribbon react with excess hydrochloric acid.

(Ar of Mg = 24)

Mg(s) + 2HCl(aq) 鈫 MgCl2(aq) + H2(g)

\(moles~of~Mg = \frac {0.3}{24} = 0.0125 mol \)

moles of H2 = 0.0125 mol (as 1:1 ratio of Mg:H2)

volume of H2 = moles 脳 Vm = 0.0125 脳 24000 = 300 cm3