Mass of Substance and Number of Particles

  1. Sometime, you may be given the mass of a substance, and asked to find the number of particles (or vice versa). 
  2. To solve the problem, we must find the number of mole of the substance then only we can find the number of particles (or mass of the substance).
Example:

How many molecules are there in 16 g of oxygen (O2)? [ Ar: O =.16, Avogadro Number = 6 x 1023]

Answer:
The relative molecular mass of  O2 = 2(16) = 32
Number of mole of oxygen molecules = Mass of oxygen Molar mass of oxygen = 16 g 32 g mol -1 =0.5 mol