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

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

Question 2
Find the mass of methane (CH4) that contain  1.806 x 1023 of methane molecules. [Relative atomic mass: Carbon: 12; Hydrogen: 1]

Answer:
The relative molecular mass of CH4
= 12 + 4(1)
= 16

 Number of mole of C H 4 = 1.806× 10 23 6.02× 10 23 =0.3 mole
Mass of CH4
 = Number of mole x molar mass 
= 0.3 x 16 = 4.8g