STANDARD MOLAR VOLUME

OF HYDROGEN

Lab #11

The purpose of this experiment is to determine the molar volume of hydrogen gas at standard conditions. To accomplish this, you will react a carefully weighed piece of magnesium metal with hydrochloric acid to produce hydrogen gas, and capture the gas in a gas measuring tube.

Procedure

1. Obtain a piece of polished Mg ribbon and weigh it on a milligram balance. It should weigh less than 0.040 g; if it is too heavy, snip some off and weigh it again. Record the mass. Fold the Mg ribbon and enclose it in a cage of fine copper wire, with a handle on the cage that will let you place the piece of Mg near the 50-mL mark in the gas measuring tube with the handle extending out the bottom, as shown in the drawing.

2. Set up a ring stand and clamp, and place a large beaker about 2/3 full of tap water nearby.

3. Dispense 15 mL 3 M HCl into the gas measuring tube. Pour tap water down the inside wall of the tube so the water forms a layer on top of the acid. Fill the tube all the way to the top.

4. Insert the stopper with the caged Mg into the top of the tube, pressing it in and letting water flow out through the holes. There should be no air bubbles.

5. Fill a large (1000 mL) graduated cylinder nearly full with tap water. To begin the reaction, cover the stopper holes with your finger, invert the tube into the graduated cylinder of tap water, and support it so it can't tip over during the reaction. The more dense acid solution will flow down toward the open end of the tube, and when it contacts the Mg metal the reaction will begin. After the reaction is finished, wait about 5 minutes to let everything come to room temperature. Observe what happens to the gas volume in the tube when you move the tube up and down in the water. Move the tube to a position where the water levels inside the tube and outside the tube match. Hold the tube there, and record the gas volume (to two places after the decimal). The purpose of doing this is to assure that you take the volume reading when the gas pressure inside the tube is the same as the air pressure outside the tube.

6. Record the room pressure (in mm Hg) and room temperature.

Analysis

1. The reaction is Mg (s) + HCl (aq)  MgCl2 (aq) + H2 (g). Balance the equation and use stoichiometry to calculate how many moles of H2 gas were produced in your reaction.

2. Convert the gas volume into liters and the temperature into Kelvins.

3. If the water levels inside and outside the tube were the same when you read the volume, the total gas pressure inside the tube is the same as room pressure. Because it was collected over water, the gas sample in the tube was a mixture of hydrogen molecules and water vapor molecules. The total gas pressure in the tube was the sum of the pressure caused by H2 molecules plus the pressure caused by water vapor molecules: Ptotal = Phydrogen + Pwater vapor. Look up the water vapor pressure at the temperature of your experiment, and calculate the H2 pressure in your gas measuring tube, in mm Hg.

4. Standard conditions for gases have been chosen to be 273 K (0 °C) and 1.00 atm (760 mm Hg), usually referred to as STP (standard temperature and pressure). Your experiment was not conducted at STP, but you can use the gas law to calculate the volume of H2 you would have obtained at STP, for the same number of moles of hydrogen.

Put your measured values of P1(use the hydrogen pressure, without water vapor), V1 and T1 into the gas law. Solve for V2 at standard pressure (P2 = 760 mm Hg) and standard temperature (T2 = 273 K).

5. Calculate the standard molar volume of hydrogen gas:

Round to appropriate sigfigs and include the unit.

Discussion

The accepted value for the standard molar volume of any gas is 22.4 L/mol. Calculate your % error and discuss the accuracy of your result, including possible sources of error.