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Formula_of_a_Copper_Chloride_Hydrate-1 - Determining the...

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1 Determining the Percent Composition and Formula of a Copper Chloride Hydrate Overview: The mass percents of Cu, Cl and H 2 O in a compound are determined by separating and massing the three components. A major emphasis of laboratory work for a chemist is determining the composition of a compound. There are many tools (such as, chromatographic separation and spectroscopy) available to aid the chemist in determining chemical composition. As you study chemistry at Wofford you will be exposed to many of these analysis techniques. In this lab you will become familiar with the techniques of sample heating, vacuum filtration, and the proper handling of strong acids. We will start with a copper chloride hydrate of unknown composition (Cu x Cl y z H 2 O). We will sequentially separate the components of water and copper, leaving behind a solution of chloride ions. The water will be removed by heating. Hydrated salts quickly lose their waters of hydration during heating. Reweighing the sample after heating will reveal the mass of anhydrous (without water) Cu x Cl y , and thus the mass of water initially in the compound. Dissolving the salt in water and reacting the copper ions with a more reactive metal will separate out the copper. The copper collected in this way is then massed. The mass of chloride present in the compound is obtained by difference. Experimental Procedure: Removing the H 2 O: Turn on the hotplate located between the desks or in the hood and adjust to a setting of 5 or 6. Clean, dry, and weigh a small ceramic crucible. Weigh out between 1 and 2 grams of the blue copper chloride hydrate into the crucible. Remember, no chemical is to be dispensed at the balance. Obtain the mass of the crucible and compound together. Heat the sample in the crucible to drive off the water. Watch the heating carefully. If the sample begins to spit as it releases water, remove the sample from the hot plate with your tongs. If the hot plate is too hot, lower the temperature. As the water is removed from the compound, the color will change from blue to brown. Continue heating until the blue color is gone. You may need to gently stir the powder with your spatula. Any compound that sticks to the spatula should be taped back into the crucible. Heat until no more color change is observed. Remove the crucible from the hot plate with crucible tongs. Let the sample cool with the lid in place. On humid days the salt will start to rehydrate from water in the air. You cannot weigh a hot object on the balance. The heat will cause convection currents over the balance, resulting in unstable readings.
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