Gravimetric Analysis of a Chloride Salt

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A common way of precipitating a chloride ion is by adding a silver ion, to create silver chloride. Ag+(aq) + Cl-(aq) → AgCl(s) Solid silver chloride (AgCl(s)) is very insoluble; however, its low solubility can be expressed by: AgCl(s) → Ag+(aq) + Cl-(aq) Ksp = 1.6x10-10 This precipitation reaction of silver chloride happens rapidly, which may cause other ions or precipitates to be present during precipitation. For example, a weak acid such as carbonate (also known as carbonic acid) could form a precipitate with excess silver ions, causing the precipitate to have a slightly higher mass, thus skewing the results of further calculations. To avoid any excess ions interfering with this process is to allow precipitation to carry out as slowly as possible in the presence of an acid solution. The acid intermediate helps prevent co-precipitation. Such quick precipitation also causes silver chloride to form into very small particles, called a colloid. To make it into a more definite form, the precipitate has to be heated and stirred gently in the presence of nitric acid. Ag+(aq) + Cl-(aq) + heat → AgCl(s) The nitric acid prevents the precipitate from reverting back to a colloid state. If the sample was to re-colloid, it would easily wash right through the filter, causing the analytical results to be low. Once the precipitate is dried, the silver and chlorine can be separated by simply being in the presence of light, illustrated in the below equation: AgCl(s) → Ag(s) + 1/2Cl2(g) This decomposition happens quite quickly in strong light; however, it mostly occurs on the surface of the precipitate because its opaqueness protects all the material below the surface. If there are excess silver ions during the decomposition, another reaction may occur: 3Cl2(g) + 3H2O(l) + 5Ag+(aq) → 5AgCl(s) + ClO3-(aq) + 6H+(aq) In this reaction, some of the chloride

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