The determination of a chemical formula lab

CHEM 403 Exp 5 Experiment 5: DETERMINATION OF AN UNKNOWN CHEMICAL FORMULA* When atoms of one element combine with those of another, the combining ratio is typically an integer or a simple fraction; 1:2, 1:1, 2:1, and 2:3 are ratios one might encounter. The simplest formula of a compound expresses that atom ratio. Some substances with the ratios listed include CaCl2, KBr, Ag2O, and Fe2O3. When more than two elements are present in a compound, the formula still indicates the atom ratio. Thus the substance with the formula Na2SO4 indicates that the sodium, sulfur, and oxygen atoms occur in that compound in the ratio 2:1:4. Many compounds have more complex formulas than those noted here, but the same principles apply. To determine the formula of a compound, one needs to find the mass of each of the elements in a weighed sample of that compound. For example, if you resolved a sample of the compound NaOH weighing 40 grams into its elements, you would obtain approximately 23 grams of sodium, 16 grams of oxygen, and 1 gram of hydrogen. Since sodium atoms have a relative mass of 23, oxygen atoms a relative mass of 16, and hydrogen atoms a relative mass of 1, you could conclude that the sample of NaOH contained equal numbers of Na, O, and H atoms. Since that is the case, the atom ratio Na:O:H is 1:1:1, and so the simplest formula is NaOH. In terms of moles, one mole of NaOH contains one mole of Na, one mole of O, and one mole of H. In this experiment, you will use these principles to find the formula of a compound called copper chloride hydrate, with the general formula CuxCly∙zH2O, where the x, y, and z are integers which, when known, establish the formula of the compound. (In expressing the formula of a compound like this one, where water molecules remain intact within the compound, the formula of H2O is retained in the formula of the compound.) This compound has a characteristic blue-green color which changes to tan or brown when the water is slowly removed by heating. The compound formed is anhydrous (no water) copper chloride. By subtracting its mass from that of the hydrate, you 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. In the next step, you will need to find either the mass of copper or the mass of chlorine in the anhydrous sample. It turns out to be much easier to determine the mass of the copper, and find the mass of chlorine by difference. This is achieved by dissolving the anhydrous sample in water, which gives a green solution containing copper and chloride ions. To that solution, an aluminum nail is added. In contact with a solution containing copper ions, the aluminum metal will react chemically with those ions, converting them to copper metal. The aluminum is said to reduce the copper ions to the metal, and is itself oxidized. The copper metal appears on the nail as the reaction proceeds, and has a red-orange color. When the reaction is complete, the copper can be separated and collected from the solution and weighed. From its mass, you can calculate the number of moles of copper in the sample. You can then find the mass of chlorine by subtracting the mass of copper from that of the anhydrous copper chloride, and from that value determine the number of moles of chlorine. The mole ratio for Cu:Cl:H2O will give you the formula of the compound. *Adapted from Slowinski, E. J., Wolsey, W. C. Chemical Principles in the Laboratory 9th ed. CHEM 403 Exp 5 Procedure Weigh a clean, dry crucible, and record its mass. Place about 1 gram of the unknown hydrated copper chloride in the crucible and record the total mass of the sample with the crucible. With your spatula, break up any sizeable crystal particles by pressing them against the wall of the crucible. Place the uncovered crucible on a clay triangle supported by an iron ring. Light your Bunsen burner away from the crucible, and adjust the burner so that you have a small flame. Holding the burner in your hand, gently heat the crucible as you move the burner back and forth. Do not overheat the sample. As the sample warms, you will see that the green crystals begin to change to brown around the edges. Continue gentle heating, slowly converting all of the hydrated crystals to the anhydrous brown form. After all of the crystals appear to be brown, continue heating gently, moving the burner back and forth judiciously, for an additional two minutes. Remove the burner, and let the crucible cool for about 15 minutes. Slowly roll the brown crystals around the crucible. If some green crystals remain, repeat the heating process. Finally, weigh the cool crucible with your dehydrated sample. Transfer the brown crystals in the crucible to an empty 50-mL beaker. Rinse out the crucible with distilled water, and add the rinsings to the beaker. You should have approximately 20 mL of water total after rinsing. Swirl the beaker gently to dissolve the brown solid. The color will change to green as the sample dissolves. Add an aluminum nail to the solution. Within a few moments you will observe some evolution of H2, hydrogen gas, and the formation of copper metal on the Al nail. As the copper ions are reduced, the color of the solution will fade. The aluminum will be slowly oxidized and enter the solution as aluminum ions.