Mole Relationships:

Mass, Volume & Pressure

By Dan Holmquist and Robyn Johnson

In celebration of Mole Day ’98, we have written an experiment that many of our chemistry teachers have requested: a CBL™/MBL experiment that emphasizes mole relationships in chemical reactions. In this experiment, students not only observe what happens to the amount of a product as the amount of one reactant increases; they also discover there can be a limit to the amount of product that can be produced due to a limiting reactant. The reaction is:

Mg_(s) + 2 HCl_(aq) -----> H2_(g) + MgCl2_(aq)

Students begin with six different lengths of magnesium ribbon. In each of the six trials, a length of magnesium is reacted with 5.0 mL of 1.0 M hydrochloric acid in a closed 125-mL Erlenmeyer flask connected to a Pressure Sensor. Instead of weighing out the small masses of magnesium, cut one longer piece of magnesium ribbon (~4 meters for eight lab stations), and then divide the total mass by the length in centimeters—our value was 0.00758 g/cm. The masses of magnesium that students use in each trial should be similar to those shown here (the lengths are based on our 0.00758 g/cm constant).

Trial	1	2	3	4	5	6
Mass of Mg	0.015 g	0.030 g	0.045 g	0.060 g	0.075 g	0.090 g
Length of Mg	~2 cm	~4 cm	~6 cm	~8 cm	~10 cm	~12 cm

Students will set out to discover what happens when they increase the mass of magnesium in each trial. Will the amount of hydrogen gas (and the resulting pressure of the closed system) increase, decrease, or remain the same when one of the reactants is increased in each trial? The mass of magnesium needed to react with all of the HCl would be:

1.0 mol HCl	×	0.0050 L	×	1 mol Mg	×	24.3 g Mg	=	1.0 cm
1L	×	1	×	2 mol HCI	×	1 mol Mg	=	0.00758 g

Students do not need to see the results of this calculation. As shown in the results below, they should discover the concept of limiting reactants for themselves. Here is a summary of the student procedure:

Obtain the first piece of magnesium ribbon for Trial 1. Record its precise length, in cm.
Add about 600 mL of room-temperature water to a 1-liter beaker. Obtain about 50 mL of 1.0 M HCl solution in a 100-mL beaker. Place the magnesium ribbon into a clean 125-mL Erlenmeyer flask.
Twist the rubber stopper snugly into the neck of the Erlenmeyer flask. It is very important that the stopper be firmly in place, so that the hydrogen gas produced in this experiment does not pop the stopper out of the neck of the flask. Close the 2-way valve on the rubber stopper assembly, but leave the blue 3-way valve of the Pressure Sensor open to the atmosphere until Step 6.
Draw precisely 5 mL of 1.0 M HCl up into the syringe. Screw the syringe onto the 2-way valve on the rubber stopper. Submerge the Erlenmeyer flask into the room-temperature water bath, so the water comes up to the neck of the flask. The temperature of the air in the flask should be the same as the temperature of the water bath (allow about 1 minute).
Set up your data collection to take a reading every 2 seconds, for a period of 300 seconds (150 points). Set up the y axis so pressure is scaled from 700 to 1100 mm Hg.
Close the blue handle on the 3-way valve of the Pressure Sensor. With the flask still submerged in the water bath, begin collecting pressure vs. time data. After about 20 seconds, open the 2-way valve directly below the syringe, squirt the HCl into the Erlenmeyer flask, then close the 2-way valve. Gently swirl the contents as the reaction proceeds. Observe both the pressure reading and the amount of magnesium ribbon that remains unreacted. When the pressure reading levels off and there is only minimal bubbling from the ribbon (or no ribbon remains), stop the data collection, or simply let the data collection end after five minutes.
Examine your data on the graph of pressure vs. time and determine the initial pressure reading, p1, for the trial (the original pressure before the HCl was added to the flask). Then determine the final pressure reading, p2, after the pressure stabilized upon addition of HCl to the magnes-ium. Calculate the pressure change, Æp = p2 - p1. Record this value. Repeat Steps 2-7 for each of the other pieces of magnesium ribbon (4, 6, 8, 10, or 12 cm). Cut each piece into several ~2-cm lengths before adding it to the flask.
After you have finished the six trials, calculate the mass of each of the six magnesium strips you used. Do this by multiplying the length times the mass per length (mass = cm X g/cm). Using Graphical Analysis, a graphing calculator, or graph paper, plot a graph of the pressure change vs. mass, for each of the six trials.

Here are sample data we collected. Note that the pressure increases during the first three trials, but remains nearly constant in the last three. Magnesium is the limiting reactant in the first three trials. The amounts are stoichiometrically "just right" in the fourth trial (see the earlier calculation). After the fourth trial, the magnesium reacts with all of the hydrochloric acid solution, leaving excess magnesium that students will observe hydrochloric acid is now the limiting reactant.

Extension: Students who have already studied mass-volume relationships in chemical reactions can calculate the pressure that should be produced, then do one or two trials to confirm the results. Instruct students to collect data as described above, then toward the end of the data collection, open the 2-way valve and pull the plunger of the syringe to the original 5-mL mark hold the plunger at the 5-mL mark, and record this pressure reading for p2. This way, the increase in pressure is due entirely to hydrogen gas. (Leaving the syringe compressed by 5 mL would increase the pressure by ~30 mm Hg.) Leave the inside of the flask wet prior to doing the experiment so water vapor pressure is kept nearly constant. Our volume was found by adding the volume in the flask (148 mL), the volume in the tubing (~4 mL), and the volume in the syringe (5 mL). The following calculation was done for our second trial (P = nRT/V):

0.0303 g Mg	×	1 mol Mg	×	1 mol H2	×	62.4 L·mm Hg	×
1		24.3 g Mg		1 mol Mg		1 K·mol
				297 K	=	147 mm Hg
				0.157 L

% error =	147-136	× 100 = 7.5%
	147

Home | Products | Technical Support | Ordering

	Vernier Software & Technology
info@vernier.com	13979 SW Millikan Way
	Beaverton, OR 97005-2886
	Ph. (503) 277-2299 Fax (503) 277-2440