Determination of an Empirical Formula Background In this lab, we will explore two different methods to determine the empirical formula of the same copper oxide salt. Both methods involve converting the salt (labeled CuxOy) to its metal form. In the first method, you will use natural gas (methane, CH4); in the second method, you will use aluminum metal. You will perform this experiment with a partner (i.e., one sample for each part). After you have removed the oxygen from each salt, you will enter your results on one of the computers in lab and the instructor will graph the results.Procedure Part I: Removing the Oxygen using Methane: Place about 0.5-2.0 g of the copper oxide salt in a large test tube that will be available in the lab. Since it would be difficult to fully remove the copper from the test tube when this experiment is completed, do not use your own test tube. Weigh the empty test tube and then weigh the test tube with your sample. Clamp the test tube at an angle, so that the mouth of the test tube does not point at anyone. Connect one end of a piece of rubber tubing to a gas inlet port and connect the other end to a piece of bent glass tubing. Place the end of the tubing into your test tube. Turn on the gas slowly and after about 2 seconds, light the end of the test tube with a match. (See the figure on the next page.) Keep the gas running over the sample for the entire experiment. After the tube is lit, set up a burner under the test tube and vigorously heat the sample until it is completely converted to copper metal. Once the copper salt has been converted to copper, turn off the burner, keeping the methane flow going and the test tube lit. Once the bottom of the test tube is cool to the touch (be careful not to burn yourself on the hot test tube), turn off the methane and weigh the test tube with the copper. Once you have determined the empirical formula of the salt, you can write a balanced equation for the reaction of the salt and methane (the other products are water and carbon dioxide). Part Il: Removing the Oxygen using Aluminum: Place a known amount of the copper oxide salt (about 0.5-2.0 g) in a 400 mL beaker and add 30 mL of 3 M hydrochloric acid. Avoid spilling the acid on your skin and avoid breathing the fumes or getting them in your eyes. Be sure to use a glass stir rod in this experiment; do not use a metal spatula in this experiment. After the solution has set for about 5 minutes, add several small strips of aluminum foil to the solution (about 0.25 g or less) to react with the copper salt. The aluminum metal produces copper metal and an aluminum salt. Once you know the empirical formula of the copper oxide salt, you can write a balanced chemical equation for this reaction. When the copper salt has reacted fully (how do you know this?), add some additional HCI to react with the excess aluminum metal. When all of the aluminum metal is gone, filter the copper metal from the solution. After the liquid has drained, rinse the copper metal with distilled water twice, then twice with acetone. Keep the samples of acetone away from any open flames (burner or test tube). Allow the paper and the copper to dry overnight. Don't forget to give the instructor your values for the mass of the starting material and the product. Question: Why must you wait for the sample to cool before turning off the methane flow over the sample? (What are you keeping out of the test tube?)
Determination of an Empirical Formula
Background
In this lab, we will explore two different methods to determine the empirical formula of the same copper oxide salt. Both methods involve converting the salt (labeled CuxOy) to its metal form. In the first method, you will use natural gas (methane, CH4); in the second method, you will use aluminum metal. You will perform this experiment with a partner (i.e., one sample for each part). After you have removed the oxygen from each salt, you will enter your results on one of the computers in lab and the instructor will graph the results.
Procedure
Part I: Removing the Oxygen using Methane: Place about 0.5-2.0 g of the copper oxide salt in a large test tube that will be available in the lab. Since it would be difficult to fully remove the copper from the test tube when this experiment is completed, do not use your own test tube. Weigh the empty test tube and then weigh the test tube with your sample. Clamp the test tube at an angle, so that the mouth of the test tube does not point at anyone. Connect one end of a piece of rubber tubing to a gas inlet port and connect the other end to a piece of bent glass tubing.
Place the end of the tubing into your test tube. Turn on the gas slowly and after about 2 seconds, light the end of the test tube with a match. (See the figure on the next page.) Keep the gas running over the sample for the entire experiment. After the tube is lit, set up a burner under the test tube and vigorously heat the sample until it is completely converted to copper metal. Once the copper salt has been converted to copper, turn off the burner, keeping the methane flow going and the test tube lit. Once the bottom of the test tube is cool to the touch (be careful not to burn yourself on the hot test tube), turn off the methane and weigh the test tube with the copper.
Once you have determined the empirical formula of the salt, you can write a balanced equation for the reaction of the salt and methane (the other products are water and carbon dioxide).
Part Il: Removing the Oxygen using Aluminum: Place a known amount of the copper oxide salt (about 0.5-2.0 g) in a 400 mL beaker and add 30 mL of 3 M hydrochloric acid. Avoid spilling the acid on your skin and avoid breathing the fumes or getting them in your eyes. Be sure to use a glass stir rod in this experiment; do not use a metal spatula in this experiment. After the solution has set for about 5 minutes, add several small strips of aluminum foil to the solution (about 0.25 g or less) to react with the copper salt. The aluminum metal produces copper metal and an aluminum salt. Once you know the empirical formula of the copper oxide salt, you can write a balanced chemical equation for this reaction.
When the copper salt has reacted fully (how do you know this?), add some additional HCI to react with the excess aluminum metal. When all of the aluminum metal is gone, filter the copper metal from the solution. After the liquid has drained, rinse the copper metal with distilled water twice, then twice with acetone. Keep the samples of acetone away from any open flames (burner or test tube). Allow the paper and the copper to dry overnight. Don't forget to give the instructor your values for the mass of the starting material and the product.
Question:
Why must you wait for the sample to cool before turning off the methane flow over the sample? (What are you keeping out of the test tube?)
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