As per Avogadro's law, equal volumes of all gases contain equal number of molecules, at a constant temperature and pressure. Therefore, equal number of molecules of any gas, must occupy the same volume, at constant temperature and pressure.
Standard Molar Volume
Standard molar volume of a gas is the volume occupied by 1 mole of any gas at 273 K and 1 atm pressure (STP). It is equal to 22.4 litres of 22,400 ml. It is the same for all gases.
S.T.P. = Standard Temperature and PressureStandard Temperature = 0oC or 273 K
Standard Pressure = 1 atm or 760 mm of mercury
Calculation of Molar Volume
Example of oxygen
Mass of 1 litre of oxygen at STP = 1.429 gMass of 1 mol of oxygen = 32 g
Volume : Mass1 litre : 1. 429 g
x : 32 g
The following table gives the relation between the Gram Molecular Weight (GMW), Number of moles, Molar Volume and the Number of particles for gases at STP.
Relationship between Various Parameters of Gases, at STP
|Gas||Molecular Formula|| GMW|
|No.Of Moles|| Molar Volume|
dm3 or l
|No.of moles in 1 mole|
|Carbon dioxide||CO2||44||1||22.4||6.023x 1023|
|Nitrogen dioxide||NO2||46||1||22.4||6.023x 1023|
|Sulphur dioxide||SO2||64||1||22.4||6.023x 1023|
Problems Based on Molar Volume
Example:Calculate the volume occupied by 3.4 g of ammonia at STP. (N=14, H=1)
Gram molecular mass of ammonia (NH3) = (1 x 14) + (3 + 1) = 14 + 3 = 17 gMass of 1 mol of ammonia = 17g
Molar volume = 22.4 litresVolume of 3.4 g of ammonia at STP = ?
Mass : Volume17 g : 22.4 litre
3.4 g : x
Volume occupied by 3.4 g of ammonia at STP =4.48 litres
Example:56 ml of carbon dioxide has a mass at 0.11 g at STP. What is the molar mass of the carbon dioxide?
Mass of 56 ml of carbon dioxide at STP = 0.11 gMass of 22400 ml of carbon dioxide = ?
Mass : Volume0.11 g : 56 ml
x g : 22400 ml
Mass of 22400 ml of carbon dioxide at STP = 44 gMolar Mass of carbon dioxide = 44 g/mole.
Example:One gram of pure sulphur dioxide has a volume of 350 ml at STP. What is the Relative Molecular Mass of sulphur dioxide?
Volume of sulphur dioxide gas = 350 ml at STPMass of sulphur dioxide gas = 1 g
Mass of one mole of sulphur dioxide = x g/moleVolume of 1 mole of sulphur dioxide = 22400 ml at STP
Mass : Volume1 : 350 ml
x : 22400 ml
Mass of 1 mole of SO2 = 64 g/mole\ Relative molecular mass of sulphur dioxide = 64.
Example:100 ml of carbon monoxide, has a mass of 0.125 g at STP. Calculate the mass of 1 mole of carbon monoxide.
Volume of carbon monoxide = 100 mlMass of carbon monoxide = 0.125 g
Molar volume = 22400 mlMass of 1 mole of carbon monoxide =?
Volume : Mass100 ml : 0.125 g
22400 ml : x g
Mass of one mole of carbon monoxide = 28 g/mole\ Relative molecular mass of carbon monoxide = 28.