Chemical Formulas

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The Determination of a Chemical Formula


Second, you will conduct a chemical reaction with the dried sample, which will produce elemental copper. By measuring the mass of copper that forms, you will have the necessary information to determine the moles of copper and chlorine in your sample, and you will be able to establish the proper chemical formula.

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In this experiment, you will



Determine the water of hydration in a copper chloride hydrate sample. Conduct a reaction between a solution of copper chloride and solid aluminum. Use the results of the reaction to determine the mass and moles of Cu and Cl in the reaction. • Calculate the empirical formula of the copper chloride compound.

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In this experiment, you will use the law of definite proportions to find the chemical formula for a hydrated compound containing copper, chlorine, and water molecules locked in the crystal structure of the solid compound. The general formula for the compound is CuxCly•zH2O, and its name is copper chloride hydrate. The letters x, y, and z represent integers that will establish the proper chemical formula for this substance. First, you will gently heat a sample of the compound to drive off the water of hydration. By measuring the mass of the sample before and after heating you can determine the amount of water in the sample and identify the value of z.



One of the challenges in finding the proper chemical formula for a compound is that there may be more than one plausible mole ratio for the elements in that compound. Dalton called this the law of multiple proportions. For example, if you were testing a compound that contained iron and sulfur, the plausible chemical formula could be FeS or Fe2S3. However, if you determine the mass of iron and the mass of sulfur present in a given mass of the compound, you will be able to establish the true chemical formula of the compound.


John Dalton was an Englishman, a teacher, and an exceptional theoretical chemist. He developed and wrote the modern atomic theory at the turn of the 19th century (documents point to 1803). He was influenced by the experiments of two Frenchmen, Antoine Lavoisier and Joseph Louis Proust. A fundamental component of the modern atomic theory is that the mole ratios of elements in a compound will be small whole numbers (law of definite proportions). The whole number mole ratio is commonly referred to as the empirical formula of a compound.

Advanced Chemistry with Vernier


Computer 1

crucible with cover crucible tongs spatula ring stand, ring, and clay triangle lab burner 50 mL beaker Büchner funnel and filter flask filter paper to fit Büchner funnel watch glass unknown solid copper chloride hydrate aluminum wire, 20 gauge 6 M hydrochloric acid, HCl, solution 95% ethanol solution distilled water wash bottle balance glass stirring rod heat lamp or drying oven

1. Obtain and wear goggles. 2. Measure and record the mass of a clean, dry crucible without cover. Obtain about 1 g of the unknown copper chloride hydrate and place it in the crucible. Use a spatula to break up any large pieces of the substance by pressing the pieces against the wall of the crucible. Measure and record the mass of the crucible with compound. 3. Set up a ring stand, ring, and clay triangle for heating the sample. Rest the crucible on the clay triangle. Set up a lab burner and ignite the burner away from the crucible. Adjust the burner to get a small flame. 4. Hold the burner in your hand and move the flame slowly back and forth underneath the crucible to gently heat the sample. Do not overheat the compound. Note the color change, from blue-green to brownish, as the water of hydration is driven out of the crystals. When the sample has turned brown, gently heat the crucible for two more minutes. 5. Remove and turn off the burner. Cover the crucible and allow the sample to cool for about...
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