Question

# Aluminium reacts with sulphuric acid to form aluminium sulphate. Write the molecular formula of aluminium sulphate.

Hint: Aluminium is a metal with the symbol $Al$. It has a silvery white appearance. Metals react with acids to liberate hydrogen gas. Similarly aluminium also reacts with sulphuric acid to liberate hydrogen gas.
We know aluminium is a metal and metals react with acid to liberate hydrogen gas. In this question metal is aluminium and acid is sulphuric acid. We have to find a formula of aluminium sulphate which is also formed as a product along with hydrogen gas. Chemical formula of aluminium is $Al$ and sulphuric acid is ${H_2}S{O_4}$ . The reaction that will take place is:
$2Al + 3{H_2}S{O_4}\xrightarrow[{}]{}A{l_2}{\left( {S{O_4}} \right)_3} + 3{H_2}$
In above reaction $Al$ is aluminium, ${H_2}S{O_4}$ is sulphuric acid, ${H_2}$ is hydrogen gas and $A{l_2}{\left( {S{O_4}} \right)_3}$ is aluminium sulphate.
Charge on aluminium ion is $+ 3$, this means it is cation and charge on sulphate ion is $- 2$, this means it is anion. To make a compound neutral which is composed of aluminium and sulphate ion we have to take two ions of aluminium so that total positive charge will be $3 \times 2 = 6$ and three ions of sulphate so that total negative charge is $2 \times 3 = 6$. Now, cationic charge is equal to anionic charge; this means compound is neutral.
So, formula of aluminium sulphate is $A{l_2}{\left( {S{O_4}} \right)_3}$