Chemistry: Law of Conservation of Mass and Law of Constant Proportions

Laws of Chemical Reactions

• The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction.

Law of Constant Proportions

• Many compounds are composed of two or more elements in the same proportions, regardless of their source or preparation method.
• The ratio of the mass of hydrogen to the mass of oxygen in water is always 1:8, regardless of the source of water.
• Similarly, in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, regardless of the method or source.

Atomic Theory

• John Dalton's atomic theory states that the smallest particles of matter are atoms, and that elements are always present in definite proportions by mass in a chemical substance.

Chemical Formulae

• A chemical formula represents the composition of a compound in terms of the symbols of the elements present.
• The formula of calcium hydroxide is Ca(OH)2, indicating two hydroxyl (OH) groups joined to one calcium atom.

Molecular Mass

• Molecular mass is the sum of the atomic masses of all the atoms in a molecule of a substance, expressed in atomic mass units (u).
• Examples of molecular mass calculations include water (H2O) and nitric acid (HNO3).

Formulae of Compounds

• The formula of sodium carbonate is Na2CO3.
• The formula of ammonium sulphate is (NH4)2SO4.

Exercises

• Write down the formulae of: • Sodium oxide • Aluminium chloride • Sodium sulphide • Magnesium hydroxide