Experiment 3: Determination of an Empirical Formula

Transcription

1 Background Information The composition of a compound is defined by its chemical formula, which gives the number ratio of the different elements in the compound. For example, water has a fixed composition of two hydrogen atoms for every one oxygen atom and therefore has a chemical formula of H 2 O. However, the composition of a compound can also be defined by a fixed ratio of masses for each different element. The proportion by mass contributed by each element in a compound is the mass percentage for that element (equation 1). mass percentage = (mass of element / total mass) 100 (1) Unlike the number ratio of elements, the mass percentages for the elements (or components) in a compound can be determined experimentally by measuring the mass of each element in a substance. Various chemical reactions and analyses can be performed on a sample of a compound to determine these elemental masses. Measured mass percentages can then be used to find the corresponding number ratio of elements in the compound, using the molar masses of the elements to convert from mass to the number of moles for each element. Note that the number ratio obtained from mass percentages will be the simplest possible ratio, so the chemical formula obtained from mass percentages is the empirical formula for the compound. In this experiment, you will determine the empirical formula of a copper chloride hydrate, Cu x Cl y zh 2 O (x, y, and z are small whole numbers), by experimentally measuring the mass percentages of copper and water in the compound. The mass percentage of water in the copper chloride hydrate will be determined by carrying out the dehydration of the compound. Heating the blue-green Cu x Cl y zh 2 O at temperatures above 100 C will remove the water from the compound, leaving behind a brown anhydrous copper chloride, Cu x Cl y (equation 2). The mass of water lost from the sample can be determined from the difference in mass before and after the dehydration. Cu x Cl y zh 2 O(s) Cu x Cl y (s) + z H 2 O(g) (2) blue-green brown The mass percentage of copper in Cu x Cl y zh 2 O can be measured experimentally by carrying out an oxidation-reduction reaction of the copper chloride hydrate with aluminum metal (eq. 3). The aluminum atoms in the metal will react with the copper cations in Cu x Cl y zh 2 O, converting them Cu x Cl y zh 2 O(s) + Al(s) Cu(s) + aluminum products (3) into copper metal. The aluminum is reducing the copper ions and is itself undergoing oxidation in the reaction. The copper metal will precipitate from solution, while the aluminum-containing products of the reaction will remain in solution (when a small amount of HCl is added). The solid copper can be separated from the excess aluminum and isolated from the reaction mixture by filtration. All of the copper ions present in Cu x Cl y zh 2 O will be converted into copper metal if the reaction with aluminum goes to completion. Measuring the mass of copper obtained

2 from the reaction after drying will therefore allow for the determination of the mass percentage of copper in the copper chloride hydrate. The experimental mass percentages of copper and water in Cu x Cl y zh 2 O can be used to calculate the mass percentage of chlorine by difference, as the mass percentages of all three components in the compound must sum to 100%. The empirical formula of Cu x Cl y zh 2 O can then be determined from these mass percentages, and the success of this empirical formula determination can be evaluated by comparison to the actual formula for the compound.

3 Experimental Procedure Eye protection must be worn at all times during the experiment. Use caution when handling the 6.0 M HCl solution, as it can burn your skin. Use caution while heating your sample in Part 2; the evaporating dish and the surface of the hot plate will be very hot. Measure and record all masses to the nearest g. Dispose of all solid waste in the trash. Dispose of all waste solutions down the drain with copious amounts of water. Part 1: Mass Percentage of Copper in Cu x Cl y zh 2 O 1. Measure the mass of a clean and dry 100 ml or 150 ml beaker. Transfer about 0.80 g of Cu x Cl y zh 2 O to the beaker and measure the mass of the beaker and sample. Record the masses on the Data and Calculations page and calculate the exact mass of Cu x Cl y zh 2 O in the beaker. 2. Dissolve the Cu x Cl y zh 2 O in about 30 ml of deionized (DI) water and add in a piece of coiled aluminum wire to the Cu x Cl y zh 2 O solution. Make sure the wire is completely immersed in the solution. Cover the beaker with a watch glass and set it aside while the reaction between Cu x Cl y zh 2 O and aluminum takes place. Gently swirl the reaction mixture every few minutes to help remove the copper metal from the aluminum wire. 3. When the reaction is complete when the blue color of the solution has completely faded add several drops of 6.0 M HCl to the solution to dissolve any solid aluminum salts present in the reaction mixture. Carefully remove all of the copper metal still attached to the aluminum wire, using a metal spatula and DI water from a wash bottle. 4. Assemble a filtration apparatus in order to isolate the copper metal from the reaction solution. Clamp a large filter flask to a ring stand and then attach the flask to the vacuum line (fitted with a vacuum trap) on the lab bench. Place a plastic Buchner funnel with a rubber seal in the filter flask. Put a piece of filter paper in the funnel and wet the paper with a small amount of DI water to ensure a good seal between the paper and the funnel. 5. With the vacuum on, decant the reaction solution into the funnel. Wash the copper metal remaining in the beaker with a small volume of DI water, breaking up any large copper pieces with your metal spatula. Transfer the copper and water wash to the funnel. Continue to rinse the beaker with DI water until all of the copper has been transferred to the funnel. 6. Turn off the vacuum and add 10 ml of 95% ethanol to the funnel, making sure to immerse all of the copper in the ethanol. Turn the vacuum back on and draw air through the funnel for a few minutes. 7. Turn off the vacuum and carefully transfer all of the copper metal to an evaporating dish from the oven. Label the dish with a marker and place the dish with copper metal into the oven for at least 10 minutes.

4 Part 1: Mass Percentage of Copper in Cu x Cl y zh 2 O (continued) 8. Remove the dish with the copper from the oven and allow the dish to cool to near room temperature. Measure and record the mass of the dish and copper. 9. Remove the copper metal from the evaporating dish and wipe the inside of the dish with a paper towel. Measure and record the mass of the evaporating dish. Calculate the exact mass of copper isolated from the reaction and calculate your group s experimental mass percentage of copper in Cu x Cl y zh 2 O. Include all of your calculations on the Data and Calculations page. Part 2: Mass Percentage of Water in Cu x Cl y zh 2 O 1. Obtain an evaporating dish from the oven and allow the dish to cool to room temperature. Measure and record the mass of the empty dish. 2. Transfer about 0.35 g of Cu x Cl y zh 2 O to the dish and measure the mass of the dish and sample. Record the masses on the Data and Calculations page and calculate the exact mass of Cu x Cl y zh 2 O in the dish. 3. Place the dish and sample on a hot plate set to half-power. Heat the sample on the hot plate until all of the green crystals of Cu x Cl y zh 2 O have been converted into the brown-colored Cu x Cl y. Once the dehydration of the sample is complete, carefully remove the evaporating dish from the hot plate and allow the dish and sample to cool to near room temperature. Do not disturb or stir the solid in the dish during the dehydration process. 4. Measure and record the mass of the dish and anhydrous Cu x Cl y. Calculate the exact mass of water lost from your Cu x Cl y zh 2 O sample in the dehydration. Use this information to determine your group s experimental mass percentage of water in Cu x Cl y zh 2 O. Include all of your calculations on the Data and Calculations page. 5. Use your group s experimental mass percentages of copper and water to calculate the mass percent of chlorine in Cu x Cl y zh 2 O. 6. Use your group s mass percentages of copper, chlorine, and water for Cu x Cl y zh 2 O to determine the empirical formula of the compound. Include all of your calculations on the Data and Calculations page.

5 Data and Calculations Part 1: Mass Percentage of Copper in Cu x Cl y zh 2 O mass of beaker: mass of beaker and Cu x Cl y zh 2 O: mass of Cu x Cl y zh 2 O used in the reaction: mass of evaporating dish and copper metal: mass of evaporating dish: mass of copper isolated from the reaction: mass percentage of copper in Cu x Cl y zh 2 O: Calculations for Part 1:

6 Data and Calculations Part 2: Mass Percentage of Water in Cu x Cl y zh 2 O mass of evaporating dish: mass of dish and Cu x Cl y zh 2 O: mass of Cu x Cl y zh 2 O used for dehydration: mass of dish and Cu x Cl y after dehydration: mass of water lost from Cu x Cl y zh 2 O: mass percentage of water in Cu x Cl y zh 2 O: Calculations for Part 2:

7 Data and Calculations Determination of the Empirical Formula: mass percentage of chlorine in Cu x Cl y zh 2 O: empirical formula of Cu x Cl y zh 2 O: Calculation of the Empirical Formula: