Question

An organic compound contains 8% oxygen and 4% sulphur by mass. The minimum possible molecular weight of the compound is:
\[
  {A.{\text{ }}400} \\
  {B.{\text{ }}200} \\
  {C.{\text{ }}800} \\
  {D.{\text{ }}1600}
\]

Answer 1

Hint: We must know that the minimum molecular weight is the molecular weight divided by the number of atoms of an element present in a molecule. So we can use the following formula:
The minimum molecular weight of a compound = $\dfrac{{100}}{{percent{\text{ }}of{\text{ }}element{\text{ }}given}} \times atomic{\text{ }}mass{\text{ }}of{\text{ }}element$
Given:
Percentage of Oxygen in a compound = 8%
Percentage of sulphur in a compound = 4%

Complete step by step solution:
As given in the question, for every \[100\;g\] of the given organic compound, \[8\;g\] of oxygen and \[4\;g\] of sulphur (S) are present.
We know that,
The atomic weights of oxygen = \[16\;g/mol\]
The atomic weight of Sulphur = \[32\;g/mol\]respectively.
As given, the compound should contain at least one oxygen atom and one sulphur atom.
So, 8 grams of Oxygen and 4 grams of sulphur are present in \[100{\text{ }}g\]of compound.
So, using unitary method,
If \[8{\text{ }}g\] of oxygen = \[100{\text{ }}g\] of compound
\[16{\text{ }}g\] of oxygen = \[x{\text{ }}g\] of compound.
So, \[16{\text{ }}g\] of oxygen will be present in $ = \dfrac{{100}}{8} \times 16 = 200g/mol{\text{ }}of{\text{ }}compound$
Similarly,
If \[4{\text{ }}g\] of Sulphur = \[100{\text{ }}g\] of compound
\[32{\text{ }}g\] of sulphur = \[x{\text{ }}g\] of compound.
So, \[{\text{32 }}g\] of sulphur will be present in $ = \dfrac{{100}}{4} \times 32 = 800g/mol{\text{ }}of{\text{ }}compound$
Hence, the minimum molecular mass of the compound is \[800{\text{ }}grams\].
Hence, the correct option is option ‘C’

Note: We must know that the molecular mass or molecular weight is the total mass of a compound. It is equal to the sum of the individual atomic masses of each atom in the molecule.