2 - Determination of an Empirical Formula

2 - Determination of an Empirical Formula - Experiment 2:...

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1 mol H 2 O 18.0148 g H 2 O 1 mol Fe 55.845 g Fe 1 mol Cl 35.453 g Cl 0.005748 0.005748 0.005748 Experiment 2: Determination of an Empirical Formula Chem 131: General Chemistry I Experiment 2 Determination of an Empirical Formula Determination of the amount of water in a hydrate by measuring the mass loss on heating. Determination of the amount of copper in a hydrate by reducing copper ions to copper metal with aluminum. Calculation of the empirical formula of a copper chloride hydrate. Page 1
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Experiment 2 - Determination of an Empirical Formula Introduction Chang: pp 67–68, 86–90, 139–140 Hydrates Many ionic compounds contain a fixed number of water molecules that are chemically bound within their crystal structures. These compounds are called hydrates ("hydro" means water in ancient Greek) and the water is called ‘water of hydration’. The crystals appear to be perfectly dry, yet when heated release a large amount of water, producing an anhydrous salt ("an" means without and "hydro" means water). The anhydrous salt often appears different from its hydrate, sometimes the two have different textures and sometimes they have different colors. Adding water to the anhydrous salt yields the hydrated salt. When expressing the formula for a hydrate, it is necessary to write the anhydrous formula for the ionic compound followed by the fixed number of H 2 O molecules. A large dot is placed between the formula and the H 2 O. For example: CuSO 4 •5H 2 O hydrated salt with water of hydration (blue crystals) CuSO 4 anhydrous salt without water of hydration (white powder) The Activity Series of Metals The activity series of metals ranks metals according to their reducing ability in aqueous solution. The more active the metal is, the better its reducing ability. A more active metal will reduce the ion of a less active metal. This type of reaction is called a single replacement or displacement reaction. One example of this type of reaction uses a copper wire and a solution of silver nitrate and results in the formation of a “silver tree.” Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) A solution containing silver ions is colorless. As the reaction proceeds, the solution turns blue, due to the formation of Cu(II) ions. As the copper displaces the silver ion, the silver solid that is formed appears to grow onto the copper wire, forming the silver tree. Determination of a Formula The simplest whole-number ratio in which atoms combine to form a compound is called the empirical formula. For example, the molecular formula of hexane is C 6 H 14 . The empirical formula for the same molecule would be C 3 H 7 showing a carbon:hydrogen mole ratio of 3:7. In this experiment, you will determine the empirical formula of copper chloride hydrate with the general formula Cu x Cl y •zH 2 O, where x, y, and z are integers. First, for a given mass of the copper chloride hydrate compound, you will determine the individual mass, in grams, of Cu, Cl, and H 2 O, present. These can then be converted to moles of each present in the sample, which will then be used
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This note was uploaded on 04/19/2011 for the course CHEMICAL 131 taught by Professor Shami during the Spring '11 term at Capitol College.

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2 - Determination of an Empirical Formula - Experiment 2:...

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