Example 1 - Mass of Substance and Number of Particles

Question:
Find the number of atoms in 34g of ammonia gas (NH3). [Relative atomic mass: Nitrogen: 14; Hydrogen: 1]

Answer:
Relative molecular mass of NH3
= 14 + 3(1)
= 17

Number of mole of ammonia = 34 17 =2 mole


Number of  NH3 molecules = 2 x 6.02 x 1023 = 1.204 x 1024


Each NH3 molecule contain 4 atoms (1 nitrogen atom and 3 hydrogen atoms).


Therefore, number of atoms
= 4 x 1.204 x 1024
= 4.816 x 1024