4 Laws of Chemical Combination

Laws of Chemical Combination

• Law of Conservation of Mass:

Matter can be neither created nor destroyed.

• Law of Definite Proportions:

A given compound always contains exactly the same proportion of elements by weight.

• Law  Proportions:

If two elements can combine to form more than one compound, then the masses of one element that combines with a fixed mass of the other element are in the ratio of small whole numbers.

• Gay Lussac’s Law of Gaseous Volumes:

When gases combine or are produced in a chemical reaction, they do so in a simple ratio by volume, provided all the gases are at the same temperature and pressure.

• Avogadro Law:

Equal volumes of gases at the same temperature and pressure should contain equal number of molecules

Dalton’s Atomic Theory

• Postulates:
• All matter is made of very tiny indivisible particles called atoms.
• All the atoms of a given element are identical in mass and chemical properties whereas those of different elements have different masses and chemical properties.
• Atoms of different elements combine in a fixed whole number ratio to form compounds.
• Chemical reactions involve reorganisation of atoms. Atoms are neither created nor destroyed in a chemical reaction.
• The laws of chemical combination could be explained by Dalton’s atomic theory.

Atomic and Molecular Masses

• Atomic mass:

• The mass of an atom
• One atomic mass unit (1 amu) = Mass equal to one-twelfth of the mass of one carbon-12 atom

1 amu = 1.66056 × 10⁻²⁴ g

• Nowadays, ‘u’ (unified mass) has replaced ‘amu’.
• Average atomic mass = ∑(Mass of isotope × Relative abundance)

• Molecular Mass:

• Sum of the atomic masses of all the elements present in a molecule
• Example − Molecular mass of CO₂ = 1 × Atomic mass of carbon + 2 × Atomic mass of oxygen

= (1 × 12.011 u) + (2 × 16.00 u)

= 12.011 u + 32.00 u

= 44.011 u

• Formula Mass:

• Sum of the masses of all the atoms present in a formula unit of a compound
• Used for compounds whose constituent particles are ions
• Example − Formula mass of sodium chloride (NaCl)

= Atomic mass of sodium + Atomic mass of chlorine

= 23.0 u + 35.5 u

= 58.5 u