Question

In an experiment, equal weights of iron and sulphur ${\text{(1}}{\text{.0g)}}$ are heated together and react to form ${\text{FeS}}$. What is the weight left unreacted?
A) ${\text{0}}{\text{.250}}$
B) ${\text{0}}{\text{.579}}$
C) ${\text{0}}{\text{.875}}$
D) ${\text{0}}{\text{.425}}$

Answer 1

This is an exothermic reaction where two elements, iron, and sulphur, react to form the compound, iron sulphide. The two solids are mixed and heated in a test tube or ignition tube and can be carried out as a class or demonstration experiment to illustrate elements, mixtures, and compounds.

Complete step by step answer:
The reaction is as follows: ${\text{Fe + S}} \to {\text{Fes}}$
${\text{1mole}}$ of iron (${\text{55}}{\text{.85g/mol}}$) reacts with one mole of sulfur (${\text{32}}{\text{.1g/mol}}$)
${\text{1}}{\text{.0g}}$ each of iron and sulfur corresponds to ${\text{0}}{\text{.01791}}$ moles and ${\text{0}}{\text{.0312}}$ moles respectively.
Thus, sulfur is present in excess and iron is a limiting reagent.
${\text{0}}{\text{.01791 - 0}}{\text{.0312 = 0}}{\text{.0133}}$ moles of sulfur will remain unreacted.
This corresponds to ${\text{0}}.{\text{0133}} \times {\text{32}}{\text{.1 = 0}}{\text{.425g}}$.

So, the correct answer is Option D.

Additional Information:
The signs that occur in this chemical reaction are the glow and a new substance which is black iron sulphide formed that can’t be separated by a magnet. With this reaction we can introduce a rule which says that when only two compounds are combined, the name of the compound ends with ‘ide’. Iron sulphides are non-stoichiometric and iron deficient. All are black and water insoluble solids.
Iron sulphides are found in nature in the form of iron-sulfur proteins.

Note: Heating a mixture of iron and sulfur causes a chemical change as the properties of iron and sulfur are entirely different from the properties of iron sulphide. Solid and molten sulfur when burnt produces sulphur dioxide which is an irritating, toxic and suffocating gas.