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Stoichiometry is a fundamental concept in chemistry that involves the calculation of reactants and products in chemical reactions. It is based on the law of conservation of mass, meaning that matter cannot be created or destroyed in a closed system. Understanding stoichiometry helps students determine how much of each substance is required or produced in a reaction.
To solve stoichiometry problems, we mostly rely on balanced chemical equations, which provide the mole ratios of the reactants and products. These ratios are essential as they guide us in determining how much of one reactant is needed to react with another, and how much product will be formed.
In the exercise given, the balanced chemical equation for the reaction between iron (Fe) and sulfur (S) to form iron(II) sulfide (FeS) is:\[ \mathrm{Fe}(s) + \mathrm{S}(s) \rightarrow \mathrm{FeS}(s) \]This equation tells us that one mole of iron reacts with one mole of sulfur to produce one mole of iron sulfide. Therefore, the stoichiometric ratio of iron to sulfur to iron(II) sulfide is 1:1:1. This proportion allows us to determine the limiting reactant, which is the reactant that runs out first in a reaction, thus limiting the amount of product that can be formed.

Molar mass is a key concept in chemistry that refers to the mass of one mole of a given substance, expressed in grams per mole (g/mol). Each element has a unique molar mass that is equal to its atomic weight listed on the periodic table.
In your exercise, the conversion from grams to moles is made possible by using the molar masses of iron (Fe) and sulfur (S). Specifically, the molar mass of iron is 55.85 g/mol, and that of sulfur is 32.07 g/mol.

• To find the moles of iron, you divide the mass of iron by its molar mass: \( \frac{5.25 \, \text{g}}{55.85 \, \text{g/mol}} \). This gives approximately 0.0941 mol of iron.
• Similarly, for sulfur, you do \( \frac{12.7 \, \text{g}}{32.07 \, \text{g/mol}} \), resulting in 0.396 mol of sulfur.

These calculations are crucial for determining which reactant is in excess and which is the limiting reactant. Understanding molar mass enables students to transition from the macroscopic scale of grams to the microscopic scale of moles, facilitating accurate predictions of reaction outcomes.

Theoretical yield is the maximum amount of product that can be generated from a given amount of reactants in a chemical reaction, under ideal conditions and with complete conversion. Calculating the theoretical yield involves using the moles of the limiting reactant along with stoichiometric ratios from the balanced equation.
In the provided exercise, since iron (Fe) was determined to be the limiting reactant, it sets the maximum amount of iron(II) sulfide (FeS) that can be produced. For every mole of Fe, one mole of FeS is formed, as indicated by the stoichiometry of the reaction. Hence, the moles of Fe directly equate to moles of FeS produced.To convert moles of FeS to grams, we use its molar mass, which is calculated by adding the molar masses of iron and sulfur:\[ 87.92 \, \text{g/mol} \quad \text{(55.85 g/mol of Fe + 32.07 g/mol of S)} \]By multiplying the moles of FeS (0.0941 mol) by the molar mass of FeS (87.92 g/mol), we find the theoretical yield of FeS to be 8.27 grams. This value represents the upper limit of product formation, assuming all of the limiting reactant is used up.