Iron(II) sulfide reacts with hydrochloric acid according to the reaction: $$\mathrm{FeS}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{FeCl}_{2}(s)+\mathrm{H}_{2} \mathrm{S}(g)$$ A reaction mixture initially contains 0.223 mol FeS and 0.652 mol HCl. Once the reaction has occurred as completely as possible, what amount (in moles) of the excess reactant is left?

Stoichiometry is the study of the quantitative relationships, or ratios, between reactants and products in chemical reactions. It's like a recipe for chemistry where ingredients (reactants) must be mixed in the right proportions to make a final product. In our textbook exercise involving iron(II) sulfide and hydrochloric acid, stoichiometry helps to determine how much of each reactant is needed and what will remain after the reaction.

Using the reactants and the balanced chemical equation, you can predict the quantitative outcomes of the reaction. For example, you can calculate how much product will form or how much of a reactant is needed to completely react with a given amount of another reactant. In particular, the equation tells us that one mole of FeS reacts with two moles of HCl. Understanding stoichiometry allows us to see that a known quantity of FeS will determine how much HCl is required for the reaction to go to completion.

A chemical reaction is a process in which substances, known as reactants, transform into different substances called products. The reaction involving FeS and HCl is a chemical change because it results in the formation of new substances: iron(II) chloride (FeCl₂) and hydrogen sulfide (H₂S).

Understanding chemical reactions is crucial for mastering limiting reactant calculations. In this specific reaction, the solid state of iron(II) sulfide (FeS) and the aqueous hydrochloric acid (HCl) interact to produce a new solid (FeCl₂) and a gas (H₂S). This process is represented by the balanced chemical equation, which ensures that the law of conservation of matter is respected, indicating that matter is neither created nor destroyed, only transformed.

The mole concept is a fundamental idea in chemistry that allows chemists to work with the submicroscopic world of atoms and molecules on a macroscopic scale. It provides a bridge between the atomic scale and the real-world scale because one mole represents 6.022 x 10²³ entities, whether they are atoms, molecules, ions, or electrons.

In the context of our textbook problem, the mole allows us to 'count' atoms and molecules by weighing them. We can then use the mole concept to calculate the reacting and remaining quantities of reactants in chemical processes. For instance, beginning with 0.223 mol FeS and 0.652 mol HCl, we're able to determine that not all the HCl will react because FeS is the limiting reactant. Knowing this, it's possible to calculate that after the reaction, 0.446 mol of HCl will remain unreacted, providing a clear picture of what is left after the reaction occurs.