AVOGADRO'S NUMBER
The number of particles (atoms, ions, molecules, formula units) present in one mole of the respective substance is called Avogadro’s number or Avogadro’s constant.
- It is denoted by NA.
- Its value is 6.022 × 1023
Example:
Na = 23g = 1mole = 6.022 ×1023 atoms.
H2O = 18g = 1mole = 6.022 × 1023 Molecules.
NaCl = 58.5g = 1mole = 6.022 × 1023 formula units.
CO32– = 60g = 1mole = 6.022 × 1023 ions.
Number of atoms of an element 
Number of molecules of compound = 
× NA
× NA
Number of formula units of ionic compounds = 
x NA.
x NA.
Number of ions = 
× NA.
× NA.
MOLAR VOLUME
The volume occupied by one mole of an ideal gas at standard temperature (0°C or 273K) and pressure (1 atm), (STP) is called molar volume.
- Its value is 22.414dm3 22414 cm3.
- It is only applicable for ideal gasses at STP.
Example:
2.016g of H2 = 1mole of H2 =6.022x1023 molecules of H2 = 22.414dm3 volume of H2 at S.T.P.
16g of CH4 = 1mole of CH4 =6.022x1023 molecules of CH4 = 22.414dm3 volume of CH4 at S.T.P.
It is very interesting to know from the above data that 22.414dm3 of each gas has a different mass but the same number of molecules.
Reason:
The reason is that the masses and the sizes of the molecules do not affect the volumes. Normally, it is known that in the gaseous state the distance between molecules is 300 times greater than their diameters.
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