Consider the unbalanced chemical equation \(\mathrm{Al}_{2} \mathrm{~S}_{3}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Al}(\mathrm{OH})_{3}+\mathrm{H}_{2} \mathrm{~S}\) If \(56.0 \mathrm{~g}\) of aluminum sulfide reacts with \(48.2 \mathrm{~g}\) of water, (a) Which is the excess reactant? (b) What mass in grams of the excess reactant remains after the reaction is complete?

Stoichiometry is the mathematical relationship between the quantities of reactants and products in a chemical reaction. It's based on the conservation of mass and the concept that atoms are neither created nor destroyed in chemical reactions. Students often encounter stoichiometry when they must calculate how much of a product is formed from given amounts of reactants or when determining the limiting and excess reactants in a chemical equation.

When working through stoichiometric problems, we use a balanced chemical equation as our 'recipe' for the reaction. This equation tells us in what ratios the molecules react and are produced. In the provided exercise, stoichiometry is the tool that helps us determine the excess reactant and its remaining mass after the reaction.

Balancing chemical equations is a fundamental skill in chemistry, as it ensures that the Law of Conservation of Mass is upheld. It involves making sure that there are equal numbers of each type of atom on both sides of the reaction arrow. A balanced chemical equation is essential for stoichiometric calculations because it gives the exact ratio in which reactants combine to form products.

The given textbook solution demonstrates how to convert an unbalanced equation to a balanced one by adjusting the coefficients of the reactants and products. Remember, we can only change the coefficients in front of the compounds to balance equations, not the subscripts within the compounds. This process may require a bit of trial and error, but it's important for understanding the quantitative aspects of chemical reactions.

Molar mass is defined as the mass of one mole of a substance. It's an essential concept for quantifying reactions in chemistry and is expressed in grams per mole (g/mol). In the context of limiting and excess reactants, we use molar mass to convert the mass of reactants to moles, allowing us to use the balanced equation to find the stoichiometry of the reaction.

As illustrated in the solution, calculating the molar mass of a compound involves adding together the atomic masses (from the periodic table) of all atoms present in the molecule, thereby allowing for a step-by-step analysis of the reaction. Without accurate molar mass calculations, it would be impossible to perform stoichiometric computations, and we could not accurately determine the limiting reactant or the excess reactant in a given reaction.