• Avogadro constant (L) is given by the equation                                                                                                                                                                                                                                                                                                  where N = number of particles n = amount of substance Avogadro constant (L) = 6.022 x 10²³ mol ^-1

• Faraday constant (F) Faraday constant is defined as the molar charge of the proton.                                      F = e L              L – Avogadro constant e – Charge of the electron Faraday constant (F) = 96500 C mol^-1

• There are various ways in which the composition can be expressed.                                                                    They are; Mass fraction                                                                                                                                                                Volume fraction                                                                                                                                                            Mole fraction                                                                                                                                                                  Mass/volume                                                                                                                                                                Moles /volume

• When the composition is expressed in terms of moles / volume it is called as concentration.

• Composition of a mixture can be expresed as a fraction

Mass fraction

Mass fraction, denoted by ‘w’ is defined as “the ratio of mass of the solute and the total mass of solute and solvent present.”

In a mixture of A and B,

Volume fraction

• Composition stated as the number of parts per million of parts (ppm) = fraction x 10⁶

• Composition stated as the number of parts per billion of parts (ppb) = fraction x 10 ⁹

Composition of substances present in very small amounts is commonly expressed in terms of parts per millions (ppm) and parts per billion (ppb).

• Density of water is 1000 g dm^-3 . Density of a dilute aqueous solution can be considered as approximately equals to the density of water. Therefore, mass of 1 dm³ of a solution = 1 kg = 1000 g = 1000000 mg For such instances, mass/ volume ratio can also be expressed in ppm.                                  As a mass fraction, 1 ppm means that 1 000 000 mg of the mixture contains 1 mg of the particular substance.

For dilute solutions, 1 ppm = 1 mg dm^-3 = 1 g cm^-3

• Empirical formula : The formula that shows the simplest whole number ratio between the number of atoms in agreement with the composition of a compound is its empirical formula                                               eg. : Empirical formula of benzene is CH

• Molecular formula : The formula that shows the exact number of atoms in a molecule of a compound is its molecular formula.                                                                                                                                                     eg : Molecular formula of benzene is C₆H₆

The ratio between the empirical formula mass and the molecular formula mass of a compound is a whole number

Molecular formula = (Empirical formula) _n