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Why does the mass of silver chloride reveal the original amount of chloride ions?

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The mass of silver chloride (AgCl) reveals the original amount of chloride ions because the process relies on gravimetric analysis, a method that determines the quantity of an analyte by measuring the weight of a stable, pure, and insoluble product.12

When silver nitrate is added to a solution containing chloride ions, they combine in a precise, stoichiometric ratio to form a white precipitate of silver chloride, which is insoluble in water. Because the chemical composition of this precipitate is known and constant, the mass of the recovered solid can be used to calculate the exact amount of chloride originally present in the sample. By isolating, drying, and weighing this precipitate, chemists can accurately determine the concentration of the chloride ions that participated in the chemical precipitation.12