Question

A student found that the sulfate ion concentration in a solution of $A{l_2}{\left( {S{O_4}} \right)_3}$ was $0.22M$ . What was the concentration of $A{l_2}{\left( {S{O_4}} \right)_3}$ in the solution?

Answer 1

Aluminium sulphate is an ionic compound consisting of aluminium and sulphate groups. From the balanced chemical equation, and the moles of sulphate group, aluminium atom, aluminium sulphate the concentration of $A{l_2}{\left( {S{O_4}} \right)_3}$ can be determined.

Complete answer:
Aluminium is a metal that can lose electrons, and undergoes oxidation. Sulphur, and oxygen atoms combine with each other to form a compound or molecule known as sulphate group with the molecular formula of $\left( {S{O_4}} \right)$ . Thus, aluminium acts as a cation, when it is involved in the bond formation with sulphate group.
Two aluminium atoms combine with three molecules of sulphate ion forming a compound known as aluminium sulphate with molecular formula of $A{l_2}{\left( {S{O_4}} \right)_3}$.
The balanced chemical equation of aluminium sulphate dissociation will be as follows:
$A{l_2}{\left( {S{O_4}} \right)_3} \rightleftharpoons 2A{l^{3 + }} + 3S{O_4}^{2 - }$
From the above balanced chemical reaction, one mole of aluminium sulphate produces three moles of sulphate ion.
Thus, the concentration of aluminium sulphate is one third of the concentration of sulphate ion.
Given that the sulfate ion has a concentration of $0.22M$
Thus, concentration of aluminium sulphate will be $\dfrac{1}{3} \times 0.22 = 0.073M$
Thus, when a student found that the sulfate ion concentration in a solution of $A{l_2}{\left( {S{O_4}} \right)_3}$ was $0.22M$ then the concentration of $A{l_2}{\left( {S{O_4}} \right)_3}$ in the solution is $0.073M$

Note:
When an ionic compound is formed, the number of electrons lost by an aluminium atom is equal to the number of electrons gained by the sulphate group. Based on this only, the dissociation reaction can be written. The dissociation clearly indicates that aluminium undergoes loss of six electrons, and sulphate group undergoes gain of six electrons.