Stoichiometry and the Mole concept

Stoichiometry mainly deals with the relative quantities of reactants and their products in a balanced chemical reaction. This is used to calculate the mass, volume, concentration amount in moles and percentage of the reactants and products. This concept is based on the law of conservation of mass that states

Mass of reactant = Mass of products

Relative Atomic Mass (Ar) → Mass of 1 atom of the element / Mass of 1/12 of a Carbon-12 atom

Relative Molecular Mass (Mr) → Mass of 1 molecule / Mass of 1/12 of a Carbon-12 atom

Relative Formula Mass → A more accurate name for relative molecular mass. e.g. Relative Formula Mass of H2O = 1 X 2 + 16 = 18


Moles is used to count in chemistry as atoms are too small and too many to be counted. An easy and accurate method of doing this is to use moles. This leads us to tell you that the unit of measurement for atoms / molecules is mole (mol).

One mole of atom consists of – 6 x 1023 particles & this constant is called as Avagadro’s constant.

For conversion of moles and number of particles formula is used:

Number of moles = number of particles / 6 x 1023

Volume of Gas

Volume of one mole of gas is 24 dm3 (r.t.p) that is equal to 6 x 1023 particles.

Molar mass of

  • An atom = Relative Atomic Mass (Ar)
  • A molecule = Relative Molecular Mass (Mr)

Mass of each element in a compound divided by its Ar gives the number of moles present in the element/ compound. The simplest ratio of the previous answer provides us the Empirical formula.formula of concentration moles and volume

Important Formulas:

  1. Number of moles = number of particles / 6 x 1023
  2. Number of moles = mass of substance (g) / relative formula mass
  3. Percentage composition by mass of an element or compound = Ar of an element x number of atoms of element in formula x 100% / Mr of compound
  4. N = relative molecular mass / Mr mass from empirical formula
  5. Number of moles of gas = mass of gas (g) / Mr of gas
  6. Moles = Concentration x volume
  7. Number of moles of gas = Volume of gas (cm3) / 24000 cm3
  8. 1dm3 = 1000cm3

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