Molecular Formula


We know that the formula of water is H2O, while that of sulphuric acid is H2SO4. Similarly, the formula for carbon dioxide is CO2, while that of ammonia is NH3 and so on. Thus, a formula is a symbolic representation of one molecule of the substance, which tells the number and kinds of atoms of various elements present in its elements. The question is— how are these formulas determined? The determination of the formula of a substance involves first the determination of its empirical formula and then the molecular formula.

Definition of Molecular Formula

The molecular formula of a compound is the chemical formula that represents the true formula of its molecule. Molecular formula denotes the actual number of atoms of different elements present in a compound molecule.

Calculation of Molecular Formula

A compound’s molecular formula can be derived from its:

  1. I) Empirical formula
  2. II) Molecular mass

The following steps are involved in determining the molecular formula:

  1. a) Empirical formula calculation from the percentage composition.
  2. b) Empirical formula mass calculation by adding the atomic masses of all the atoms present in the empirical formula.
  3. c) Compound’s molecular mass determination based on the data provided.
  4. d) Determination of value of ‘n’ by using the relation, n= Molecular mass/Empirical Formula Mass
  5. e) Determining the molecular formula using the relationship:

Molecular formula = n × Empirical formula

Calcium Nitrate

Calcium nitrate is an inorganic compound with the formula Ca(NO3)2. It is known as calcium dinitrate in IUPAC nomenclature. It is also known as Norgessalpeter, tetrahydrate, Ca(NO3)2. 4H2O is formed when the anhydrous compound Ca(NO3)2 absorbs moisture from the air. Both the anhydrous and hydrated forms are colourless salts.

Physical Properties of Calcium Nitrate
  1. It is colourless to white or light grey granular powder.
  2. It is hygroscopic in nature.
  3. It has a melting point of about 560oC.
  4. It is solid at room temperature.
  5. It is soluble in methanol, acetone and anhydrous ammonia and almost insoluble in nitric acid.
Chemical Properties of Calcium Nitrate
  1. Calcium nitrate breaks down to release nitrogen dioxide(NO2) and oxygen(O2) when heated.

The following reaction takes place:

Ca(NO3)2 + heat → CaO + 2NO2 + O2

  1. NH4NO3(ammonium carbonate) and CaCO3 ( calcium carbonate) are formed when calcium nitrate reacts with ammonia, carbon dioxide, and water.

Ca(NO3)2+ 2NH3 + CO2 + H2O →  2NH4NO3 + CaCO3

Uses of Calcium Nitrate
Application in agriculture

Calcium nitrate is primarily used in fertilisers. Certain plant diseases are also controlled with calcium nitrate. Dilute calcium chloride and calcium nitrate sprays, for example, are used to control cork spots and bitter pits in apple trees.

Wastewater treatment

Calcium nitrate is used to prevent odour emissions in wastewater pre-conditioning. The wastewater pre-conditioning process is based on creating an anoxic environment in the wastewater system. In the presence of nitrate, sulphate metabolism stops, preventing the formation of hydrogen sulphide(H2S). Furthermore, easily degradable organic matter is consumed, which would otherwise cause anaerobic conditions downstream as well as odour emissions.