EXPERIMENT 13: GAS STOICHIOMETRY

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1 EXPERIMENT 13: GAS STOICHIOMETRY PURPOSE To collect a gas produced in a reaction and compare the volume actually collected to a "target volume". To discover and compensate for assumptions made in the stoichiometric calculations to collect the "target volume" of gas. BACKGROUND Magnesium is the least dense structural metal. Because of its lightness it is often alloyed with aluminum and used to make custom-designed racing car wheels called MAG wheels. Both magnesium and aluminum are very reactive with acids such as the hydrochloric acid used in this experiment. This explains why manufacturers of these expensive wheels warn consumers that the use of any acid-cleaning product will affect the surface and void the warranty. In this experiment you will carry out a reaction between magnesium and hydrochloric acid. The balanced equation for the reaction is: Mg (s) + 2 HCl (l) MgCl 2(aq) + H 2(g) The goal is to collect a specified volume of H 2 gas by calculating the mass of Mg needed to produce that volume. You will calculate how much magnesium is needed by using stoichiometry and the Ideal Gas Laws. Keep in mind, however, that the conditions in the laboratory are probably not Standard. You will need to correct for any non-stp condition. Also be aware that other gases, notably water vapor, will contaminate your hydrogen gas and you will need to account for the partial pressure of the water vapor in your product. Information about the partial pressure of water vapor can be found in your textbook.

2 MATERIALS Thermometer Barometer 5 gallon plastic bucket Gas collecting tube Four beam balance Magnesium ribbon Ruler Copper wire 1 hole rubber stopper 6 M HCl PROCEDURE 1. Find the mass of exactly 30 cm of magnesium ribbon (your instructor may provide this information). 2. Calculate the mass of magnesium metal needed to collect exactly 40 ml of H 2 gas from the reaction given in the Background. 3. Calculate the length of magnesium ribbon having the necessary mass. Cut the needed length of magnesium ribbon and fold it into a small ball no larger than a pea. 4. Obtain about 25 cm of copper wire and wrap it around the Mg ribbon. As the acid reacts with the Mg ribbon, the ribbon will become smaller and smaller. Be sure that you wrap the copper wire tightly enough so that the magnesium ribbon will not fall out as it decreases in size. Be sure to leave a "tail" of copper wire free ( about 3-5 cm) when you are finished wrapping. Your instructor will show you what this should look like when you are finished. 5. Pour 10 ml of 6M HCl into the gas collecting tube. Tilt the tube to about 45 and VERY GENTLY pour water into the tube until it overflows. The object here is to have a layer of water on top with no acid mixed in. As you pour the water over the top, watch for the tell-tale signs of mixing. If you are careful, you will "see" the mixing for only a few cm. Note : It is important that steps 6 and 7 be done quickly: 6. Insert the copper wire cage into the water so that the "tail" is outside of the tube. Fit the stopper into the tube so that the copper cage is fixed in place. 7. Fill the stopper hole with water. Put a finger over the hole, invert the tube and insert the tube into the 5 gallon bucket filled with water. Remove your finger from the rubber stopper. 8. When the reaction has stopped, raise your tube out of the water until the height of the water inside the tube is the same as the height of the water in the bucket. Record the volume of gas inside the tube. Remember to read the volume at the bottom of the meniscus. 9. Find and record the temperature of the water in the bucket, the temperature of the air in the room and the barometric pressure in the lab.

3 Name: Date PRE-LAB QUESTIONS 1. When the tube is inverted, how does the magnesium react with the acid if the magnesium is at the bottom of the tube and the acid is at the top? Explains what happens and why. 2. How do you know that the reaction has stopped? 3. When reading the gas volume in the tube, what is the purpose of equalizing the water level in the bucket with the water level in the tube? 4. Write the balanced chemical reaction that occurs. 5. Calculate the mass of magnesium metal needed to collect exactly 80 ml of H 2 gas from the reaction above. Assume standard conditions.

4 Name: Date: GAS STOICHIOMETRY 1. Mass of 30 cm of Mg ribbon 2. Mass of Mg needed to produce 40 ml H 2 3. Length of Mg ribbon 4. Volume of 6M HCl 5. Volume of gas produced 6. Water temperature 7. Room temperature 8. Atmospheric pressure CALCULATIONS AND RESULTS 1. What volume of gas were you expecting to collect? 2. Calculate the percent of error between the volume of gas you actually collected and the volume of gas that you were expecting to collect. 3. Which assumption made in your calculations produced the biggest source of error? 4. With the mass of Mg ribbon used, recalculate the volume that you should expect to collect correcting for the error above.

5 5. Calculate the new percent error between the new expected value and the actual volume collected. 6. Although the chemical reaction in this experiment produces only H 2 gas, there was another gas present in the tube. What was it? 7. Why was this gas there? 8. Recalculate your expected volume, accounting for the presence of this "extra gas". 9. Calculate the new percent error between the new expected value and the actual volume collected. 10. What other sources of error could produce this new percent error?