The mole & the Avogadro Constant

Chemical amounts are measured in moles

• The mole, symbol mol, is the SI unit of amount of substance

  
  

• One mole of a substance contains the same number of the stated particles

  • This can be atoms, molecules or ions 

    
    

• One mole contains 6.02 x 10^23 particles; this number is known as the Avogadro Constant

• For example:

  
  

  • One mole of sodium (Na) contains 6.02 x 10^23 atoms of sodium

    
    

    • One mole of hydrogen (H2) contains 6.02 x 10^23 molecules of hydrogen

      
      

    • One mole of sodium chloride (NaCl) contains 6.02 x 10^23 formula units of sodium chloride

• The mass of 1 mole of a substance is known as the molar mass

  • For an element, it is the same as the relative atomic mass written in grams

  • For a compound, it is the same as the relative molecular or formula mass in grams

The mole & volume of gas

Molar volumes of gas

• Avogadro’s Law states that at the same temperature and pressure, equal amounts of gases occupy the same volume of space

  • e.g. 1 mole of hydrogen gas occupies the same volume as 1 mole of methane gas

• At room temperature and pressure, the volume occupied by one mole of any gas was found to be 24 dm3 or 24,000 cm3

  • This is known as the molar gas volume at RTP

  • RTP stands for “room temperature and pressure” and the conditions are 20 ºC and 1 atmosphere (atm)

• From the molar gas volume, the following formula triangles can be derived:

This shows the relationship between moles of gas, volume in dm3 and the molar volume

If the volume is given in cm3 instead of dm3, then divide by 24,000 instead of 24

The formula can be used to calculate the number of moles of gases from a given volume or vice versa

• Simply cover the one you want and the triangle tells you what to do

• For example, to find the volume of a gas:

  • Volume = Moles x Molar Volume