# The Formula Of A Compound

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1 The Formula Of A Compound Introduction: A compound is a distinct substance composed of two or more elements chemically combined in fixed proportions. Atoms of the elements in a compound are combined in whole number ratios, not fractional parts. For example, water is represented as H 2 O. This formula means 2 hydrogen atoms are combined with 1 oxygen atom. The ratio of hydrogen to oxygen is always 2 to 1 in the compound we call water, never 2.07 to 1. (Who proposed this?) Compounds need not have any properties in common with the elements from which they are made. At room temperature, both hydrogen and oxygen are gases, and a mixture of the two is potentially explosive. Water, on the other hand, is a liquid at room temperature and is not explosive. It is impossible to prepare and weigh a quantity of matter as small as 2 hydrogen atoms and 1 oxygen atom using ordinary laboratory procedures. Instead, the quantity most frequently used when describing amounts of matter is the mole. Think of a mole as the chemical version of a dozen. A dozen always represents the same number regardless of what is being measured. A mole represents a number as well. This number, referred to as Avogadro's number, is very large ( = 1 mole). The formula for water, H 2 O, represents 2 hydrogen atoms combined with 1 oxygen atom to form 1 molecule of water. It would be just as true to say that the formula could represent 2 dozen hydrogen atoms combined with 1 dozen oxygen atoms to form 1 dozen water molecules. With this in mind, does it make sense to say the formula could also represent 2 moles of hydrogen atoms combined with 1 mole of oxygen atoms to give 1 mole of water molecules? In this experiment sulfur is allowed to chemically react with copper to form a new compound. Any excess sulfur left over is burned away. From the data collected the following will be determined: Procedure: 1. The percent of sulfur and copper in the compound. 2. A chemical formula for the compound. CLEANING THE CRUCIBLE In this experiment, we will heat the crucible red-hot for several minutes and then use the sample s mass change to determine the chemical formula. Before we start, we want to remove any chemical dirt from the crucible, that is, anything that would evaporate when the crucible is red-hot. Carefully inspect the crucible for cracks. Clean the crucible by wiping the inside with a paper towel (don't use water). Place the crucible on a clay triangle, heat to redness, and allow it to cool to room temperature. Weigh the cool crucible, but do not weigh the cover. Carefully inspect the crucible for cracks again. Most cracking takes place when a wet crucible is heated too rapidly (or dropped). At this point you have invested significant time & effort into your crucible breaking it would cost you valuable lab time. If you are not completing the experiment on the same day, store it carefully in your draw.

2 FIRST REACTION CYCLE Obtain a sample of copper from the stockroom. The weight of the copper will be given to you later. Examine it, making note of color and flexibility. This means that you should record observations in your lab notebook! Place the copper in your crucible and take it to the stockroom for some sulfur. Examine the sulfur, making note of the color and physical appearance. The crucible and contents are covered with the lid, and then heated slowly with a small flame in the hood. This causes the sulfur to melt and boil. Some of the boiling sulfur combines with the hot copper to form a new compound. In some cases, the excess sulfur escapes from the crucible as vapor and burns below the lid. If the vapors ignite, remove the burner. Continue heating and burning of the sulfur until no blue flames are seen coming from under the lid. After the sulfur stops burning, the intensity of heating is increased until the bottom part of the crucible becomes red hot for about 2 minutes. The crucible is allowed to cool to room temperature on the clay triangle. Keep the lid on the crucible during the heating and cooling operations. The crucible cover is used to reduce loss from spattering and to keep oxygen in the air from reacting with the contents. After the crucible has cooled to room temperature it can be weighed again without the cover. SECOND REACTION CYCLE Cover the product in the crucible with more sulfur, then heat, cool, and weigh as before. This cycle is repeated until the mass does not change by more than g. Now the reaction is complete and all of the copper has been converted to the sulfide. When done, examine the product, making note of the color, flexibility and physical appearance then discard it in the jar provided. Clean the crucible and cover by heating in a hot flame. Go to the stockroom and obtain a slip of paper that will have the mass of the copper. Data and Observations Data should be recorded in tabular form in your notebook. Include all information on weighing and physical properties.

3 Calculations From your data, calculate the following in your notebook. Observe significant figures: Obtain the mass of the copper metal from the stockroom. Calculate the mass of the product in the crucible when you were finished. Calculate the mass of sulfur that was used to make the product. Calculate the percentage of sulfur in the product. Calculate the moles of copper used in the reaction. Calculate the moles of sulfur used in the reaction. Calculate the moles of copper that react with each mole of sulfur; that is: moles Cu per moles S. Now write two chemical formulas for the compound: FORMULA (1): a chemical formula for the compound using your exact experimental data. FORMULA (2): a chemically sensible formula for the compound. Results Report the following in your lab notebook: 1. Show how each calculation was performed (neatly show your work). 2. Compare the physical properties of the product with those of the reactants. Does the product resemble the copper? The sulfur? Explain why it should or shouldn't. 3. If you broke the product apart would you expect the inside to still look like the copper? Explain why you would or wouldn't. Conclusion: Report the experimental and empirical formulas. Explain if and how the purpose was fulfilled. Explain why the experimental formula had to be "adjusted" to be chemically sensible. If your results do not match what we know about copper and sulfur (ionic compounds, known charges), propose an explanation.

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5 Prelab: Name: Complete the following calculations in your lab notebook. Clearly identify each answer. After you finish, rewrite prelab calculations (showing work) on this sheet to turn in before lab starts. A student measured the following in this experiment: Mass of cleaned crucible: g Mass of Copper: g Mass of crucible & compound after second heating: g Determine the following using these numbers (be sure to report sig. figs. & units!): 1. Calculate the mass of the product in the crucible when you were finished. 2. Calculate the mass of sulfur that was used to make the product. Assume that no copper was lost during the experiment. mass S 3. Calculate the percentage of sulfur in the product. % = 100 mass compound 4. Calculate the moles of copper used in the reaction. 5. Calculate the moles of sulfur used in the reaction. 6. Calculate the moles of copper that react with each mole of sulfur; that is: moles Cu moles S 7. Experimental formula of the compound (don t round off) & Empirical formula of the compound.

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