Gas Volumes: At the same temperature and pressure, equal volumes of different gases contain
same number of molecules.
• Molar Gas Volume: Vm. It is the volume per mole of gas molecules. Volume of gas depends on
pressure and temperature.
• RTP: At RTP, 1 mole of gas molecules has a volume of 24.0 dm3 = 24000 cm3
. Therefore assume
the molar gas volume is 24.0 dm3 mol-1
.
• Calculation- Moles: Moles = volume/ molar gas volume. Moles = Volume/ 24 dm3 mol-1
. cm3
/
1000 = dm3
. Volume aka ‘volume of gas collected/ produced’.
• Ideal Gas Equation: Gas equation used when need to be more accurate or when carrying out
experiment not at RTP.
• pV = nRT.
– R always 8.314 J mol-1 K
-1
. n is moles- use moles of gas- or find using molar ratio.
– Volume is in m3
. Cm3
to dm3
to m3
, multiply or divide by 1000 each time.
– Pressure is in Pa. kPa to Pa is divide by 1000.
– C to K is +273.
• Assumptions for Ideal Gas: For an ideal gas, the assumptions for the molecules are
– Random motions
– Elastic collisions.
– Negligible size.
– No intermolecular forces.
• Experiment to Find Mr: Can use ideal gas equation to find Mr of volatile liquid. Add volatile liquid
to small syringe. Weigh small syringe. Inject sample into gas syringe. Reweigh small syringe to find
mass of volatile liquid added to gas syringe. Place gas syringe in boiling water bath. Liquid
vaporises to gas.
