Chemical formula is a symbolic representation of a molecule’s composition. It indicates –
(a) the elements that are present in the compound and
(b) the ratio of these elements in the compound.
A chemical formula is also called molecular formula. The symbols in the molecular formula tell us about the elements and the subscript tells us how many atoms of that element are present in one molecule.
For example, a molecule of water (H2O), consists of two hydrogen atoms and one oxygen atom. It is therefore represented by the chemical formula H2O. Lack of a subscript on the O indicates one atom of O per water molecule. Sodium chloride (common salt) contains equal number of sodium and chlorine atoms and is represented by the formula, NaCl. Sulphuric acid, H2SO4 contains three elements – hydrogen, oxygen and sulphur in ratio 2 : 4 : 1.
Read more –
What is Matter? What is its Characteristics?
Problems based on Law of Conservation of Mass
Law of Definite Proportions / Constant Composition
Problems based on law of definite proportions or constant composition
Mole Concept – Importance and formulas
Rules for writing a chemical formula
- Write down the symbols of the elements / ions, which combine to form a molecule of the compound, side by side. While writing the formula of a compound containing a metal and a non-metal, the symbol of the metal is written first followed by that of the non-metal. Two metals do not join up to form compounds.
- Write the valency of each element / ion.
- Interchange the valencies of the elements / ions and write as the subscript. Write them close together and ignore ‘1’ to obtain the formula.
- The radical / polyatomic ions must be written in brackets, before the subscript is written. In case the number of polyatomic ion is one, the bracket is not required. For example – NaOH.
- Whenever applicable, write simple ratios of the valencies.
- The final formula must be written without any sign of the charge.
The following examples will help you to acquire better knowledge about writing the formulae of compound.
- Water contains the elements hydrogen and oxygen.
(a) Write the symbol. H O
(b) Write the valency of each element 1 2
(c) Interchange the valencies of the elements and write as the subscript. H2 O1
(d) Write them close together and ignore ‘1’ to obtain the formula. H2O - Hydrogen sulphide contains the elements hydrogen and sulphur.
(a) Write the symbol. H S
(b) Write the valency of each element 1 2
(c) Interchange the valencies of the elements and write as the subscript. H2 S1
(d) Write them close together and ignore ‘1’ to obtain the formula. H2S - Hydrogen chloride contains the elements hydrogen and chlorine.
(a) Write the symbol. H Cl
(b) Write the valency of each element 1 1
(c) Interchange the valencies of the elements and write as the subscript. H1 Cl1
(d) Write them close together and ignore ‘1’ to obtain the formula. HCl - Carbon tetrachloride contains the elements carbon and chlorine.
(a) Write the symbol. C Cl
(b) Write the valency of each element 4 1
(c) Interchange the valencies of the elements and write as the subscript. C1 Cl4
(d) Write them close together and ignore ‘1’ to obtain the formula. CCl4 - Carbon dioxide contains the elements carbon and oxygen.
(a) Write the symbol. C O
(b) Write the valency of each element 4 2
(c) Interchange the valencies of the elements and write as the subscript. C2 O4
(d) Write simple ratios of the valencies. C1 O2
(e) Write them close together and ignore ‘1’ to obtain the formula. CO2
Example 1 – An element A combines with oxygen. The valency of A is 3. What will be the formula?
Solution–
(a) Write the symbol. A O
(b) Write the valency of each element 3 2
(c) Interchange the valencies of the elements and write as the subscript. A2 O3
(d) Write them close together to obtain the formula. A2O3
Example 2 – An element A forms its chloride and sulphide. The valency of A is 2. What will be the formula of its chloride and sulphide?
Solution–
Chloride of A –
(a) Symbol. A Cl
(b) Valency of each element. 2 1
(c) Interchanged valencies in the subscript. A1 Cl2
(d) Formula. ACl2
Sulphide of A –
(a) Symbol. A S
(b) Valency of each element 2 2
(c) Interchanged valencies in the subscript. A2 S2
(d) Formula. AS
Example 3 – An element A shows variable valencies 2 and 3. What will be the formula of its oxides with two valencies?
Solution–
Oxide of A when its valency is 2-
(a) Symbol. A O
(b) Valency of each element. 2 2
(c) Interchanged valencies in the subscript. A2 O2
(d) Formula. AO
Oxide of A when its valency is 3-
(a) Symbol. A O
(b) Valency of each element. 3 2
(c) Interchanged valencies in the subscript. A2 O3
(d) Formula. A2O3
Example 4 – An element A forms an oxide A2O5. Find out its valency. What will be the formula of its bromide?
Solution–
Oxide of A – A2O5
(a) Symbol. A O
(b) Number of atoms present in the molecule. 2 5
(c) Valency of each element. ? 2
(d) Valency of 5 oxygen atoms 5 x 2 = 10
In the compound A2O5, the total valencies of 2 atoms of A should be equal to the valencies of 5 atoms of oxygen i.e. 10. Therefore, the valency of A element is 5.
Bromide of A when its valency is 5-
(a) Symbol. A Br
(b) Valency of each element. 5 1
(c) Interchanged valencies in the subscript. A1 Br5
(d) Formula. ABr5
Next –
Rules for Writing Chemical Formulae of Ionic Compounds
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