Question

Equal masses of $ {S_2},{S_4},{S_8} $ are taken. Highest number of sulphur atoms are present in?
A) $ {S_2} $
B) $ {S_4} $
C) $ {S_8} $
D) All have equal number of atoms

Answer 1

Hint: Sulphur is present in various allotropic forms like Rhombic Sulphur, monoclinic sulphur, colloidal sulphur etc. The molecules $ {S_2},{S_4},{S_8} $ are just different molecules of sulphur having the same molecular mass.

Complete answer:
The atomic mass of sulphur is found to be 32 grams per mol. Consider that you are given 1 mol each of $ {S_2},{S_4},{S_8} $ . Therefore, all the three forms $ {S_2},{S_4},{S_8} $ will also have the same atomic mass of 32g/mol.
To find the total number of sulphur atoms, we will divide the atomic mass by Avogadro’s number $ {N_A} $ which is equal to $ 6.022 \times {10^{23}} $
Since the atomic mass of all the molecules $ {S_2},{S_4},{S_8} $ are the same, the no. of atoms present in them is also the same. The difference between the compounds given to us is that they differ in the no. of molecules of sulphur they constitute. Therefore, every form of sulphur ( $ {S_2},{S_4},{S_8} $ ) will differ in the no. of atoms of sulphur in the whole molecule.
Consider the form $ {S_2} $ . Here the no. of molecules is the same i.e. $ 32/6.022 \times {10^{23}}molecules $ . These molecules are divided among only two atoms of sulphur. Hence each atom of sulphur will have more molecules compared to other forms.
The other form $ {S_8} $ has the same no. of molecules ( $ 32/6.022 \times {10^{23}}molecules $ ). These molecules are divided among 8 atoms of sulphur. Hence the no. of molecules per atom of sulphur will be least.
The increasing order of atoms of sulphur can be given as: $ {S_8} < {S_4} < {S_2} $
The correct answer is Option (A).

Note:
Remember that for any element, present in different forms (differing in no of molecules that constitute them) the total no. of molecules of the element in the compound will be the same. The form having least no. of atoms of the element will have the highest no. of molecules. Don’t get confused in the terms of atoms and molecules.