Gas Laws
Boyle’s Law
The volume of a given mass of a dry gas is inversely proportional to its pressure at a constant temperature. P
1V
1 = P
2V
2 = k at constant temperature
Charles’s Law
The volume of a given mass of a dry gas is directly proportional to its absolute temperature if the pressure is kept constant.
at constant pressure.Gas Equation
The volume of a given mass of a dry gas is inversely proportional to the pressure and directly proportional to the absolute temperature.
Standard or Normal Temperature and Pressure
- For temperature: 0°C or 273 K
- For pressure: 760 mm or 76 cm of Hg
Gay-Lussac’s law of combining volumes
At the same temperature and pressure, the volume of gases taking part in a chemical reaction as either reactants or products bears a whole number ratio to one another.
Example:
The ratio of reacting gases and products is 1:1:2, which is a simple ratio.Avogadro’s Law
Under the same conditions of temperature and pressure, equal volumes of all the gases contain the same number of molecules.
Example: A molecule of NH3 is made of one atom of nitrogen and three atoms of hydrogen.

Atomicity
The number of atoms in a molecule of an element is called its atomicity.
- Monatomic: It is composed of only one atom. Examples: Inert gases such as Helium, Neon etc.
- Diatomic: It is composed of two identical atoms. Examples: H2, O2, Cl2 etc.
- Triatomic: It is composed of three identical atoms. Example: Ozone (O3)
- Tetratomic: It is composed of four identical atoms. Example: Phosphorus (P4)
- Octatomic: It is composed of eight identical atoms. Example: Sulphur (S8)
Atomic Mass or Relative Atomic Mass
It is the number which represents how many times one atom of an element is heavier than 1/12
th the mass of an atom of carbon-12 (
12C).

Molecular Mass or Relative Molecular Mass
It is the number which represents how many times one molecule of an element is heavier than 1/12
th the mass of an atom of carbon-12 (
12C).

Gram Atomic Mass
The atomic mass of an element expressed in gram is called gram atomic mass.
Example: Gram atomic mass of oxygen is 16 gram.
Gram Molecular Mass
The molecular mass of a substance expressed in gram is called gram molecular mass or molar mass.
Example: Gram molecular mass of water is 18 gram.
Mole Concept
A mole is a collection of 6.022 × 10
23 particles.
A mole is defined as the amount of a substance containing elementary particles such as atoms, molecules or ions in 12 gram of carbon-12 (
12C).
Avogadro’s Number
It is defined as the number of atoms present in 12 gram of C-12 isotope, i.e. 6·023 × 10
23 atoms. It is denoted by NA or L.
NA = 6·023 × 10
231 mole of atoms = 6·023 × 10
23 atoms
1 mole of molecules = 6·023 × 10
23 molecules
1 mole of electrons = 6·023 × 10
23 electrons
1 mole of a gas = 22·4 litre at STP
Applications of Avogadro’s Law
- It explains Gay-Lussac’s law.
- It determines atomicity of the gases.
- It determines the molecular formula of a gas.
- It determines the relation between molecular mass and vapour density.
- It gives the relationship between gram molecular mass and gram molar volume.
Relative Vapour Density (VD)
Relative vapour density is the ratio between the masses of equal volumes of a gas (or vapour) and hydrogen under the same conditions of temperature and pressure.
Relative molecular mass of a gas or vapour = 2 × VD

Percentage Composition
The percentage by weight of each element present in a compound is called percentage composition of the compound.

Empirical Formula
It is the chemical formula which gives the simplest ratio in whole numbers of atoms of different elements present in one molecule of the compound.
Empirical Formula Mass
It is the sum of atomic masses of various elements present in the empirical formula.
Empirical Formula Weight (EFW)
The empirical formula weight is the atomic masses of the elements present in the empirical formula.
EFW of H
2O
2 = 2 × (H) + 2 × (0)
= 2 × 1 + 2 × 16
= 34 amu
Molecular Formula
It denotes the actual number of atoms of different elements present in one molecule of the compound.
Molecular formula = Empirical formula × n

Relationship between Empirical Formula and Molecular Formula
Molecular formula = Empirical formula × n
Where ‘n’ is a positive whole number

Chemical Equation
A shorthand notation of describing an actual chemical reaction in terms of symbols and formula along with the number of atoms and molecules of the reactants and products is called a chemical equation.
A chemical equation is a balanced account of a chemical transaction.
