Chemistry 101 Generating Hydrogen Gas

Objectives • To experimentally verify the molar volume of hydrogen gas at STP • To gain experience in collecting gas over water Discussion The molar volume of a gas is the volume occupied by one mole of gas. At STP (Standard Temperature, 0ºC, and Pressure, 1 atm), all ideal gases have a molar volume of 22.4 L, regardless of the identity of the gas. In this experiment, we will test this using hydrogen gas, generated by the reaction of magnesium with hydrochloric acid:

Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g) As demonstrated in lecture, the reaction will be run underwater, and the gas will be captured using an inverted graduated cylinder as the gas displaces water in the cylinder (see Figure 3 below). The molar volume = VSTP/n, where n is the moles of hydrogen and VSTP is the volume of hydrogen at STP. The volume of hydrogen gas generated by the reaction, Vlab, can simply be read from the graduated cylinder. However, because the laboratory is not under STP conditions, you need to “correct” Vlab to the volume it would occupy under STP conditions (VSTP):

From the ideal gas law, 1 1 2 2

1 2

P V P VT T

=

Assign one set of variables to be in the lab conditions, and the other to be STP:

lab lab STP STP

lab STP

P V P VT T

=

Finally, solve for VSTP (our goal): lab lab STPSTP

STP lab

P V TV

P T∗ ∗

=∗

;

So, in order to calculate VSTP, you need five quantities. TSTP and PSTP are easy – they come from the definition of STP, given above. The other three come from measurements: Tlab will be measured from a thermometer. Vlab will be the volume of hydrogen gas you read from the graduated cylinder, when the reaction is complete. Plab is not so straightforward. The gas collected in the top of the cylinder has a pressure equal to the atmospheric pressure in the laboratory, which can be measured by a barometer in the lab. For your lab session, this value will be written on the white-board – you won’t need to read the barometer yourself. The problem is that the gas in the cylinder is not pure hydrogen gas, so we can’t just assign this to be Plab. Instead, we need to subtract out the pressure of the water vapor: PH2 = Ptot – PH2O. Ptot is the atmospheric pressure in the laboratory and equal to the pressure of the gas mixture in the cylinder; PH2 is what we will call Plab. The partial pressure of water (PH2O) depends only on the temperature, according to the following table:

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Table 1 Vapor Pressure of Water

Temperature oC Pressure mm Hg Temperature oC Pressure mm Hg 16 14 24 22 17 15 25 24 18 16 26 25 19 17 27 27 20 18 28 28 21 19 29 30 22 20 30 32 23 21

For example, if the pressure in the room is 757.6 mm Hg and you conduct the reaction at 23.0ºC: Plab = PH2 = 757.6 – 21 mm Hg = 737 mm Hg (remember addn/sub rule for s.f.!) At last, you can calculate VSTP and then the molar volume can be determined (VSTP/n). Moles of hydrogen are calculated using the balanced chemical equation and the mass of magnesium. See below for an example.

2H2 2

1 mol H1 mol Mgn 0.0791g Mg 0.00325 mol H24.3050g Mg 1 mol Mg

⎛ ⎞⎛ ⎞= =⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠

Procedure 1. Work in groups of two students unless otherwise instructed. 2. Obtain a plastic trough, a large funnel, and a 100 mL graduated cylinder from the CHE101 lab counter. Return this equipment clean when finished with the lab. Fill the trough about 2/3 full of water. Place the cylinder horizontally into the water and allow it to fill completely with water. There should be no air bubbles in the cylinder. 3. Weigh one Mg strip on an analytical balance, and record its mass on the data sheet. 4. Prepare the Mg for reacting by coiling it loosely. Wrap it up in a small, single ply square of cheesecloth and close it all up with a piece of copper wire. The cheesecloth and copper will not react with the acid – they are used to weigh down and contain the Mg. This “wrapping” of the Mg will be demonstrated in lecture.

Figure1 Mg strip lightly coiled, and wrapped and tied with a copper wire. 5. Measure about 50mL of 6 M HCl solution into a 100 mL beaker.

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6. You’re ready to conduct the reaction. Place the Mg “packet” underwater in the small, slanted indentation at the bottom of the trough.

Figure 2 Use the indentation on the left of the trough Stand the graduated cylinder upright (bottom up) so that the mouth is directly over the Mg. Trap part of the cheesecloth under the graduated cylinder as shown in Figure 1. This keeps the cheesecloth from floating up as it reacts. There should be no air bubbles in the cylinder. One student will holds the graduated while the reaction occurs. Position the tip of the funnel so that the acid will be delivered as directly as possible to the Mg. Begin by adding about 25 mL of HCl slowly through the funnel. You should see bubbles immediately start to form, and these bubbles should be completely captured within the graduated cylinder. When the reaction slows, add another portion of acid, and repeat until all the Mg has disappeared and no more bubbles are being formed.

Figure 3 Gas Collection Setup 7. Read the volume of hydrogen gas. Record this volume as Vlab on your data sheet, and remember to record the proper number of significant figures. 8. Repeat steps # 1 through 7 two more times. Reuse the copper wire. 9. During one of the trials, measure the temperature of the water, in Celsius, and record it as Tlab. The temperature of the water will probably not change dramatically throughout the course of the experiment. 10. Do your calculations for each trial and check with lab staff before leaving.

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DATA AND CALCULATIONS

Trial 1 2 3 a. Mass of Mg (g)

b. Temperature of water (°C)

c. Temperature of water (K) = Tlab

d. Volume of hydrogen generated (mL) in lab conditions = Vlab

Atmospheric pressure in room during experiment (Ptot) = ______________ mm Hg (from white-board) Calculate PH2 for each trial: PH2 = Ptot – PH2O

Trial 1 2 3 e. PH2O (from Table 1 and Tlab)

f. PH2 (referred to as Plab in next calculation)

Calculate the STP volume of hydrogen, using: lab lab STPSTP

STP lab

P V TV

P T∗ ∗

=∗

(Show your work, for one

trial, below)

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Trial 1 2 3 g. Volume of hydrogen at STP (in L) = VSTP

Calculate moles of H2 gas generated, n, based on mass of Mg and balanced chemical equation (see example). Show work for one trial here:

Trial 1 2 3 (Optional)

h. Moles hydrogen (n)

Finally, calculate the molar volume of hydrogen at STP ( = VSTP/n):

Trial 1 2 3 (Optional i. Molar volume of hydrogen at STP(L/mole)

Calculate the average from the trials: _________ L/mole Calculate a percent error for your average: _________ % Error __________ │literature –experimental value │ x 100 literature value