Determination of a Chemical Formula

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CHEM 1105

Experiment 4:
Determination of a Chemical Formula

When atoms of one element combine with those of another, the combining ratio is typically an integer or a simple fraction. The simplest formula of a compound expresses that atom ratio. When two or more elements are present in a compound, the formula still indicates the atom ratio. To find the formula of a compound we need to find the mass of each of the elements in a weighed sample of that compound. For example, if we resolved a sample of the compound NaOH weighing 40 grams into its elements, we would find that we obtained just about 23 grams of sodium, 16 grams of oxygen, and 1 gram of hydrogen. The sample of NaOH contains equal numbers of Na, O, and H atoms. Since this is the case, the atom ratio Na:O:H is 1:1:1, and so the simplest formula is NaOH. In terms of moles, we have one mole of Na, 23 grams, one mole of O, 16 grams, and one more of H, 1 gram. From this kind of argument we can conclude that the atom ratio in a compound is equal to the mole ratio. We get the mole ratio from chemical analysis, and from that the formula of the compound. In this experiment, we will use these principles to find the formula of the compound with the general formula CuxCly zH2O, where x, y, and z are integers which establish the formula of the compound. The compound we will study is called copper chloride hydrate. We first drive out the water, which is called hydration. This occurs if we gently heat the sample to a little over 100*C. The compound formed is anhydrous (no water) copper chloride. If we subtract its mass from that hydrate, we can determine the mass of the water that was driven off, and using the molar mass of water, find the number of moles of H2O that were in the sample. Next, we need to the find either the mass of copper of chlorine in the anhydrous sample we have prepared. (It is easier to find one mass and find the other by difference.) We do this by dissolving the...
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