Molar Volume Of A Gas 3b4y6i

Selena Yasmin Sapphire Galadriel Seay-Reynolds Due: February 16, 2011

Molar Volume of a Gas When magnesium metal reacts with hydrochloric acid, hydrogen gas is produced. The volume of this gas can be measured by using a eudiometer, a closed-ended glass tube calibrated for volume readings. Knowing the number of moles of magnesium used, as determined from the mass of the magnesium reacted in excess acid, one can calculate the volume of hydrogen produced per mole of magnesium consumed. The balanced equation for this reaction allows determining the volume that one mole of gas would occupy at a specific temperature and pressure. Magnesium is the least dense structural metal. Because of its lightness, it is often alloyed with aluminum and used to make custom designed racing wheels, called MAG wheels. Both magnesium and aluminum are very reactive with acids, such as with the hydrochloric acid used in this experiment. This explains why manufacturers of these very expensive wheels warn consumers that the use of any acidic cleaning product will affect the surface and void the warranty. Adapted in part from the Laboratory Manual for Chemistry, 4th ed, published by Addison-Wesley ©1995.

Materials 400 mL beaker 50 mL beaker thermometer 1000 mL graduated cylinder ring stand buret clamp or utility clamp

50 mL eudiometer centimeter ruler one-hole rubber stopper magnesium ribbon thread 6 M hydrochloric acid

Procedures 1. Fill a 400 mL beaker two-thirds full of water. If possible, use water that has adjusted to room temperature.

Obtain a piece of magnesium ribbon from your instructor; the length of this ribbon should measure less than 2.3 cm and be recorded to the nearest .01 cm. Record this length in your data table. Also be sure to get the value of the mass of one meter of ribbon from your instructor and record this value as well. 2. Roll the length of magnesium ribbon into a loose coil. Tie it with one end of a piece of thread, approximately 25 cm in length, in such a manner that all the loops of the coil are tied together. 3. This next procedure requires the use of 6 M HCl. Carefully pour approximately 10 mL of 6 M HCl into a 50 mL beaker. Then pour the 10 mL of 6 M HCl into the eudiometer. 4. While holding the eudiometer in a slightly tipped position, very slowly pour water from the 400 mL beaker into the eudiometer, being careful to layer the water over the acid so that they do not mix. Add enough water to fill the eudiometer to about 5 cm from the top of the tube. 5. Lower the magnesium coil into the water in the eudiometer. Finish filling the eudiometer so that it is completely full of water. 6. Insert the one-hole rubber stopper into the open end of the eudiometer to hold the thread in position. The one-hole stopper should displace some water from the tube. This ensures that no air is left inside the tube. 7. Cover the hole of the stopper with your finger and invert the eudiometer in the 400 mL beaker of water. Clamp the eudiometer into position on the ring stand using the buret clamp. The acid flows down the tube and reacts with the magnesium. Describe your observations. 8. When the magnesium has disappeared entirely and the reaction has stopped, cover the stopper hole with a finger and carefully transfer the eudiometer to a 1000 mL graduated cylinder. Adjust the level of the eudiometer such that the levels of the liquids inside and outside the eudiometer are the same. Read as accurately and precisely as possible to determine the total volume of gas. 9. Record the room temperature and pressure. 10. Obtain the vapor pressure of water at the observed temperature from a standard source, such as the CRC Handbook of Chemistry and Physics. Record this value in your data table. Wash your hands. 1

Selena Yasmin Sapphire Galadriel Seay-Reynolds Due: February 16, 2011

Data and Calculations Length of Mg used Mass per meter of Mg Volume of H2 collected under lab conditions Temperature of collected H2 Barometer reading Vapor pressure of water at observed temperature

2.28 cm 1.28 g/m 29.64 mL 21.7°C 767.6 mm Hg 19.587 mm Hg

Observations: When we turned the eudiometer upside down, and the HCl and H2O began to mix, the HCl slowly sunk down from the top, swirling on its way down. As we watched, the magnesium rose in the tube. Bubbles, slowly at first, but then increasingly rapidly formed and rose. The volume of HCl and H2O in the eudiometer quickly began to decrease, and continued to fall, at increasing speed. Not much change appeared to occur in the magnesium while this was occurring, apart from the bubbles that formed and rose from it, but after the volume of HCl and H2O in the tube was close to half-way down, the reaction suddenly stopped, and the solid magnesium was gone. The bubbles and almost all motion within the tube stopped, and the beaker and eudiometer were cool to the touch. 1. Write a balanced equation for the reaction of magnesium with HCl to form hydrogen gas and aqueous

magnesium chloride. Mg (s) + 2HCl (aq) → H2 (g) + MgCl2 (aq) 2. Determine the number of moles of hydrogen gas that your sample of magnesium ribbon should have created. atomic mass Mg = 24.305 amu; molar mass Mg = 24.305 g/mol 2.28 cm Mg x 1m x 1.28 g = 0.0292 g Mg 100 cm 1m 0.0292 g Mg x 1 mol Mg = 0.00120 mol Mg 24.305 g Mg 0.00120 mol Mg x 1 mol H2 = 0.00120 mol H2 1 mol Mg 3. Since the hydrogen was collected over water, two gases were actually present: hydrogen and water vapor. Calculate the partial pressure of the hydrogen gas collected. 767.6 mm Hg – 19.587 mm Hg = 748.0 mm Hg 4. Calculate the volume the collected dry hydrogen gas (i.e. using the partial pressure of hydrogen) would occupy if the conditions were changed to STP. (748.0 mm Hg)(29.64 mL x 1 L/1000 mL) = (760 mm Hg)(V2) (21.7 + 273.15 K) (273.15 K) V2 = 0.02702 L H2 5. Calculate from your data the ratio of the volume to the number of moles of dry hydrogen gas at STP. This is your experimental molar volume. 0.02702 L H2 = 22.5 L/mol H2 0.001120 mol H2 6. What is the theoretical value of the molar volume of the hydrogen (assuming ideal behavior) at STP? 22.4 L/mol H2 7. What is your percent error? 22.5 L/mol H2 – 22.4 L/mol H2 x 100% = 0.400% 22.4 L/mol H2

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Selena Yasmin Sapphire Galadriel Seay-Reynolds Due: February 16, 2011

Conclusions and Questions 1. In this lab it is necessary to make a water-vapor correction of the barometer reading. Why must we calculate

the partial pressure of hydrogen? Why can’t we just use the total pressure in the system? In our calculations, we needed to use the combined gas law to reach our final answers. The combined gas law is derived from the ideal gas law, and is therefore based on and functions on an assumption of ideal behavior. Water vapor is not an ideally behaving gas, as it is below its boiling point. It cannot be used in either the ideal gas law or combined gas law. In addition, we had no way of finding the moles of water vapor present in the system, for other calculations. 2. From the balanced equation, determine the theoretical volume of dry hydrogen gas at STP that can be produced from 3.00 moles of magnesium reacting with excess acid. 3.00 mol Mg x 1 mol H2 = 3.00 mol H2 1 mol Mg 3.00 mol H2 x 22.4 L = 67.2 L H2 1 mol 3. Since the 1930's, aluminum/magnesium alloys have been used in the manufacture of pots and pans. An unanswered question is whether the small addition of these metals to our food supply is beneficial, harmful, or of no consequence. (For instance, some studies seem to indicate a connection between aluminum and Alzheimer's disease.) What are some examples of foods that will react with aluminum/magnesium pans? In this experiment, the reaction occurred between magnesium and hydrochloric acid. Hydrochloric acid is a relatively strong acid, but weaker acids will still react with magnesium. Therefore, acidic foods and ingredients will react with magnesium/aluminum pans. Examples include tomatoes, vinegar, lemon juice, and pickles.

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