A 2.5624-g sample of a pure solid alkali metal chloride is dissolved in water and treated with excess silver nitrate. The resulting precipitate, filtered and dried, weighs 3.03707 g. What was the percent by mass of chloride ion in the original compound? What is the identity of the salt?

Stoichiometry is a branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It allows us to predict the amounts of substances consumed and produced in a reaction. In the context of our problem, stoichiometry is used to determine the mole ratio between the alkali metal chloride and the silver chloride, which is 1:1.

For every mole of alkali metal chloride (MCl) that reacts, one mole of silver chloride (AgCl) precipitate is formed. This mole-to-mole conversion is essential in calculating the percent by mass of the chloride ion in the original sample. By using stoichiometric calculations, we determine the moles of chloride ions from the moles of AgCl precipitate—which is a direct stoichiometric equivalent because of the 1:1 ratio. Understanding stoichiometry is key in solving many chemical problems, particularly in determining compositions of compounds and reactants.

Gravimetric analysis is a method of quantitative chemical analysis where the amount of a substance is determined by the mass of a solid. In our problem, it involves dissolving a sample of unknown alkali metal chloride and reacting it with silver nitrate to form a precipitate of silver chloride. The mass of the precipitate is then measured after it has been filtered and dried.

The mass of the precipitate directly relates to the amount of chloride present in the original sample since each chloride ion forms one AgCl unit. By analyzing the mass of the precipitate, we can calculate the moles of the compound and consequently its percent by mass. Gravimetric analysis is not only precise but also a fundamental tool for identifying the composition of substances in a mixture.

Calculating the molar mass, which is the mass of one mole of a substance, is a fundamental step in quantitative chemistry. It helps in relating the mass of a substance to the number of moles and atoms or molecules it contains. In our exercise, we use the molar mass of silver chloride (AgCl) and chloride ion (Cl-) to convert between mass and moles.

By knowing the molar mass of these substances, we can calculate the moles of AgCl precipitated and, in turn, the moles and mass of chloride ions in the original sample. The molar mass of a substance is typically found by adding the atomic masses of the constituent elements sourced from the periodic table. This calculation is crucial for any chemical quantitative analysis, including the determination of percent compositions.

Precipitation reactions are vital in analytical chemistry as they allow for the separation of a particular ion from a solution. When an ionic compound is insoluble in water, it forms a solid precipitate, as seen in the reaction between an alkali metal chloride and silver nitrate to produce silver chloride.

The creation of the silver chloride precipitate is a visual indicator of the reaction's progression and completion. The mass of this precipitate can then be used to deduce the quantity of the chloride ions present in the original sample. Understanding reaction stoichiometry and the solubility of compounds are essential to effectively utilize precipitation reactions in quantitative analysis, such as determining the percent composition of an element within a compound.