Question

Write the chemical formula of
1) calcium sulphate
2) Aluminium phosphate
3) Aluminium sulphate

Answer 1

Hint: It a shows the elements which the compound contains and the numbers of atoms of each element in one mole of that compound. There are some rules of writing the chemical formulae of the compounds. Chemical formulae can be said as symbolic representations of chemical compounds.

Complete answer:
Binary compounds made up of two different elements only. Formulae of such compounds can be written using valencies since both types of atom forming the compound must have lost or gained the electrons.
Let us see the rules of writing chemical formulae then we will apply these to write them.
(1) Determine the valencies of the two elements present. Valency is the number of chemical bonds that an element can form.
(2) Write the symbols of elements and chemical formula, so that the sum of the valencies of the two elements are equal. Do this by crossing the charge.
(3) If a metal is present, always place it first in the formula.
Let us start writing by compound 1) Calcium sulphate: The symbol of calcium is $\text{Ca}$ and sulphate is $\text{SO}_{4}^{2-}$. The valency of calcium is 2 as its configuration is $\left( 2,8,8,2 \right)$. The charge on sulphate is -2. Now using the crossing the charge method, the chemical formula will be

The formula of calcium sulphate will be $\text{C}{{\text{a}}_{2}}{{\left( \text{S}{{\text{O}}_{4}} \right)}_{2}}$ which in simplest form will be $\text{CaS}{{\text{O}}_{4}}$.


2) Aluminium phosphate: The symbol of Aluminium is $\text{Al}$ and phosphate is$\text{PO}_{4}^{3-}$ . The valency of aluminium is 3 as its configuration is $\left( 2,8,3 \right)$. The charge on phosphate is -3. Now using the crossing the charge method, the chemical formula will be

The formula of aluminium phosphate will be $\text{A}{{\text{l}}_{3}}{{\left( \text{P}{{\text{O}}_{4}} \right)}_{3}}$ which in simplest form will be $\text{AlP}{{\text{O}}_{4}}$.

3) Aluminium sulphate: The symbol of Aluminium is $\text{Al}$ and sulphate is $\text{SO}_{4}^{2-}$. The valency of aluminium is 3 as its configuration is $\left( 2,8,3 \right)$. The charge on sulphate is -2. Now using the crossing the charge method, the

The formula of aluminium sulphate will be $\text{A}{{\text{l}}_{2}}{{\left( \text{S}{{\text{O}}_{4}} \right)}_{3}}$.

Note: After knowing what chemical formulae are, the next question that comes to our minds is; why is there a need for chemical formulae? The answer to this question is
(1) To write chemical equations all the time and it would take too long to write and read, if we use words. So, it is easier to use symbols. Each element has a symbol. The particular way of writing using symbols is called a chemical formula.
(2) In chemical formulae, all the elements that formed the compound and use a small number to the bottom right of an element’s symbol to show the number of atoms of that element are present in the compound. Like the chemical formula for water is ${{\text{H}}_{2}}\text{O}$. It represents a water molecule that has two hydrogen atoms and one oxygen atom.