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.