By: Howard Andres

In this experiment, our lab group’s purpose was to find the formula of a compound with the general formula of Cu_xCl_y   zH₂O, where the x, y, and z are integers which, when known, establish the formula of the compound.  To find the formula of the compound, we needed to find the mass of each of the elements in a weighed sample of that compound.  Then, by converting these masses into moles, and comparing their ratios with one another, we would be able to determine the formula of the compound.

Several observations were made during the experiment, especially with the color of the compound.  The hydrated sample was a blue-green color, and when heated so to drive off the water, the remaining of the compound turned to a tan-brown color.  Once that dehydrated sample was put into a solution of water there was a greenish color, and when the aluminum wire was added into it, a reaction occurred that resulted with clumps of red-orange copper metal forming around the wire.  A gas also evolved during the reaction.  Once the reaction was complete we were to separate the copper from the wire, and it was somewhat easily broken off with the help of the stirring rod and 6 M HCl, leaving the wire looking pretty much in the same condition as when we first received it, a greyish color.  We additionally observed that when we tried to scrape all of the copper off the filter paper, residue remained that could not be taken off, which we thought could affect our results.

Through the data we received during the experiment we were able to determine the masses of each component of the Cu_xCl_y   zH₂O compound.  Then by converting these masses to moles we were able to compare them in ratios with the copper in the sample.  The ratio of chlorine to copper was 3.9:1, and the ratio of water to copper was 3.1:1.  By rounding these integers, we found the formula of the dehydrated sample to be CuCl₄, and the formula of the hydrated sample to be CuCl₄   3H₂O.

There is probably error to this though.  One possible error that caused it is that not all of the 10 mL of 95% ethanol was able to be added into the funnel during the suction process, which may have had an affect upon our results.  Also, the sample probably did not dehydrate all of the way when it was heated, which as a result probably affected the mass of the sample, making it greater than it should be.  Another possible error is with not being able to put all of the copper into the watch glass for the final mass measurement.  Despite for the most part being easy to scrape off the aluminum wire, not all would, leaving copper residue on it.  Residue was also left on the filter paper and the spatula that was used to scrape it since not all of it could be taken off.  As a result, the mass in the final weight measurement of the copper is probably a little less than what it ought to be with some of the copper being left on the aluminum wire, filter paper, and spatula.  Such errors thus most likely explain why the formula of the compound is CuCl₄   3H₂O.  Copper usually has a +1 or +2 charge, but this equation shows copper having a +4 charge, with four chlorines with a -1 charge necessary to make the compound have a 0 charge.  The several possible errors most likely explain such a result, affecting the masses of the components of the compound, which affects their mole ratios, and thereby affects the formula of the compound.

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