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One mole of any gas (or the formula mass in g), at the same temperature and pressure occupies the same volume. This is 24dm3 (24 litres) or 24000 cm3, at room temperature and pressure (r.t.p).

Equal volumes of all gases contain the same number of molecules which is 02 X 1023 molecules, while V= 24000 cm3 at room temperature and pressure (r.t.p), or 22400 cm3 at standard temperature and pressure (s.t.p).

When gases combine, they do so in small volumes which bear a simple ratio to one another and to the volume of product if gaseous. All volumes must be measured at the same temperature and pressure

Example. Given the equation MgCO3(s) + H2SO4(aq) ==> MgSO4(aq) + H2O(l) +CO2(g). What mass of magnesium carbonate is needed to make 6 dm3 of carbon dioxide at r.t.p?

[Ar’s: Mg = 24, C = 12, O = 16, H =1 and S = 32]

Since 1 mole = 24 dm3, 6 dm3 is equal to 6/24 = 0.25 mol of gas

From the equation, 1 mole of MgCO3 produces 1 mole of CO2, which occupies a volume of 24 dm3

Molecular mass of MgCO3 = 24 + 12 + 3×16 = 84, So to calculate mass of MgCO3 = mol x molecular mass
= 0.25 x 84 = 21g

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