How do you Write Chemical Formula for an Ionic Compound?

Rules for writing a chemical formula

1. 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.).
2. Write the valency of each ion.
3. 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.
4. 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.
5. 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.

Law of Conservation of Mass

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?

States of Matter

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Formula Unit Mass or Formula Mass