Question

An organic compound contains 4% sulphur. Its minimum molecular weight is :
(A) 200
(B) 400
(C) 800
(D) 1600

Answer 1

Hint: Try to recall that atomic weight of sulphur is 32 g and the presence of sulphur in the organic compound is tested by Lassaigne's test. Now, by using this you can easily find the correct option from the given ones.

Complete step by step answer:
In the above question, it is mentioned that the mass of sulphur is 4 % w/w in the organic compound. This means that in every 100 g of the organic compound, the weight of sulphur is present in 4 g.
We will now try to calculate the minimum molar of the organic compound.
The minimum moles of sulphur atom present in the organic compound is 1. So, the minimum mass of sulphur present in the compound is 1 mole i.e. 32 g.
We also know that the weight percentage of sulphur is 4.
So, 4 % of the total mass of the organic compound is 32 g.
So, the minimum molar mass of organic compound = $\dfrac{100\text{ x 32}}{4}$ = 800 g.
So, the correct answer is “Option C”.

Additional Information:
In any organic compound if sulphur is present, then on fusion with sodium metal, sodium sulphide is formed and then by tests like sodium nitroprusside test, lead acetate test we can easily detect the presence of sulphur.

Note: It should be remembered to you that in sodium nitroprusside test, a small portion of the Lassaigne’s filtrate is treated with a few drops of sodium nitroprusside solution when a violet colouration is obtained.
Also, you should remember when a given organic compound is fused with a mixture of potassium nitrate and sodium carbonate. If sulphur is present, it gets oxidised to sulphate.