Rules for writing a chemical formula
- Write down the symbols of the ions (cation and anion) involved. Write the symbol of cation first (L.H.S) followed by the symbol of anion (R.H.S.).
- Write the valency of each ion.
- Interchange the valencies of the ions and write as the subscript. Write them close together and ignore ‘1’ to obtain the formula. Write the formula removing all charges.
- The radical / polyatomic ions must be written in brackets, before the subscript is written. For
example – Ca(OH)2.
In case the number of polyatomic ion is one, the bracket is not required. For example – NaOH. - Whenever applicable, write simple whole number ratio of the valencies that give a net charge of zero.
The final formula must be written without any sign of the charge.
Read more –
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
Problems / Numerical based on Mole Concept
How to write chemical formula of a simple Compound
Example 1 –
Write a chemical formula for a compound that contains Potassium ions and Bromide ions.
Solution –
(a) Symbols of the ions involved. K Br
(b) Valency of each ion. +1 -1
(c) Interchange valencies in the subscript. K1 Br1
(d) Formula. KBr
Example 2 –
Write a chemical formula for a compound that contains magnesium ions and bromide ions..
Solution –
(a) Symbols of the ions involved. Mg Br
(b) Valency of each ion. +2 -1
(c) Interchange valencies in the subscript. Mg1 Br2
(d) Formula. MgBr2
Note that there are two bromide ions (Br-) for each magnesium ion (Mg+2) in magnesium bromide so that positive and negative charges must balance each other and the overall structure must be neutral.
Example 3 –
Write a chemical formula for aluminium oxide.
Solution –
(a) Symbols of the ions involved. Al O
(b) Valency of each ion. +3 -2
(c) Interchange valencies in the subscript. Al2 O3
(d) Formula. Al2O3
Example 3 –
Write a chemical formula for calcium oxide.
Solution –
(a) Symbols of the ions involved. Ca O
(b) Valency of each ion. +2 -2
(c) Interchange valencies in the subscript. Ca2 O2
(d) Formula. CaO
Example 5 –
Write a chemical formula for sodium nitrate.
Solution –
(a) Symbols of the ions involved. Na NO3
(b) Valency of each ion. +1 -1
(c) Interchange valencies in the subscript. Na NO3
(d) Formula. NaNO3
Example 6 –
Write a chemical formula for calcium hydroxide.
Solution –
(a) Symbols of the ions involved. Ca OH
(b) Valency of each ion. +2 -1
(c) Interchanged valencies in the subscript. Ca (OH) 2
(d) Formula. Ca(OH) 2
Example 7 –
Write a chemical formula for sodium nitrate.
Solution –
(a) Symbols of the ions involved. Na NO3
(b) Valency of each ion. +1 -1
(c) Interchanged valencies in the subscript. Na NO3
(d) Formula. NaNO3
Example 8 –
Write a chemical formula for ammonium sulphate.
Solution –
(a) Symbols of the ions involved. NH4 SO4
(b) Valency of each ion. +1 -2
(c) Interchanged valencies in the subscript. (NH4) 2 SO4
(d) Formula. (NH4) 2SO4
What is Matter? What is its Characteristics?
Next –
Formula Unit Mass or Formula Mass
Pingback: WHAT IS FORMULA UNIT MASS OR FORMULA MASS? – Freakgenie
Pingback: ATOMS AND MOLECULES – NECRT TEXT BOOK Questions Solutions – Freakgenie