AS-Level Amounts of Substance

  • The ideal gas equation is used to calculate the moles of gas present in a system if the volume  (m ), pressure (Pa) and temperature (K) are known.

    • pV = nRT

      • n = moles

      • R = gas constant, 8.31 Jmol K 

  • The equation assumes that the same number of moles of all gases occupies the same volume at a set temperature and pressure.

  • At room temperature (293K and 101KPa), it is assumed that 1 mole of any gas occupies a total volume of 24000cm (24dm ).

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QUICK NOTES

Calculating Moles of a Gas

 

The same general principle applies when finding the number of moles of a gas as with liquids and solids, but there are other considerations we need to take into account.

 

Pressure and temperature control the number of moles found in a given volume of gas. The higher the temperature of a set number of moles of gas, the more volume those moles will want to occupy. The higher the pressure acting on an amount of gas, the less volume that amount of gas will occupy.

 

 

 

This can be linked to moles and volume using the ideal gas equation: pV = nRT.

 

where: p = pressure (Pa), V = volume (m ), n = number of moles, R = gas constant and T = temperature (in Kelvin)

 

A few things to point out,

 

Pressure is in pascals (Pa not KPa – a particular favourite for examiners is to give you the pressure in a question as KPa, but you must convert to Pa for the ideal gas equation. Divide by 1000).

 

Volume is in m not dm (again, divide dm by 1000 to get m ).

 

Temperature is in Kelvin. To convert from degrees Celsius to Kelvin, simply add 273.

 

We call this equation the ideal gas equation, as we make the assumption that all gases behave in the same way and occupy the same volume in space if they are subject to the same environment (temperature and pressure).

 

At room temperature and pressure, we make the assumption that 1 mole of ANY gas occupies 24000cm of space. From this we can easily calculate how many moles of that gas we may have.

 

 

 

 


For example, if a sample of Cl gas has a volume of 24 000cm at room temperature and pressure, there is 1 mole of Cl molecules.

Therefore, if a sample of Cl gas has a volume of 12 000cm at room temperature and pressure, there is 0.5 mole of Cl molecules.

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