- Sometime, you may be given the mass of a substance, and asked to find the number of particles (or vice versa).
- 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