Question

The molecular formula of aluminium sulphate is:
(A) $\text{A}{{\text{l}}_{\text{2}}}{{\text{(S}{{\text{O}}_{\text{4}}}\text{)}}_{\text{3}}}$
(B) $\text{AlS}{{\text{O}}_{\text{4}}}$
(C) $\text{A}{{\text{l}}_{3}}{{\text{(S}{{\text{O}}_{\text{4}}}\text{)}}_{\text{3}}}$
(D) $\text{A}{{\text{l}}_{\text{2}}}{{\text{(S}{{\text{O}}_{\text{4}}}\text{)}}_{4}}$

Answer 1

Hint: Chemical structure of an organic compound represents the actual number of atoms, nature of bond (saturated or unsaturated) and functional group present in one molecule of the compound i.e., it shows the real formula of its one molecule.

Complete answer:
Three valence electrons are present in the valance (outermost) shell of the aluminium atom. So this atom can make only three bonds by sharing the three valence electrons. Sulphur dioxide is the molecule of sulphur in which it binds with two oxygen atoms by two double bonds. Sulphur has six electrons in their valence shell, so after using the four electrons, with the bonding with two oxygen atoms they still have two valence electrons in the valence shell. So, two aluminium atoms can bind with the three sulphur dioxide molecules.
In an ionic bond formation equal number of charges shared by the both cation and anion.
(A) In this compound two aluminium atoms binding with the three sulphur dioxide molecules and two aluminium atoms share six electrons in the covalent bonding with the six electrons of sulphur dioxide. So this molecular formula is correct.
(B) In this an aluminium atom shares three electrons with the two electrons of the sulphur atom, which is not possible, so this structure is not possible.
(C) In this compound six electrons of aluminium atom is shared with the four atom electrons of sulphur dioxide, this is not possible.
(D) In this compound nine electrons of aluminium atoms are shared with the eight electrons of sulphur dioxide, so this compound is not possible.

So, option (A) will be the correct answer.

Note:
Covalent bond is formed by the equal sharing of electrons between the two atoms, while ionic bond is formed by the unequal sharing of electrons between the two atoms. In ionic bonds, the negative charge of the donor atom must be equal to the positive charge of the acceptor atom.