Name Date _

19 Molar Volume of a GasPRE-LAB DISCUSSION

Avogadro's hypothesis states that equal volumes of all gases contain e,gual numbers ofmolecules under the same conditions of temperature and pressure. It follows from thishypothesis that all gas samples containing the same number of molecules will occupy thesame volume under the same conditions of temperature and pressure. A special name isgiven to the volume occupied by I-mole samples of gases at STP. This volume is called themolar volume. In this experiment, you will make an experimental determination of themolar volume.

The basis of this experiment is the following reaction in which you will react a knownmass of magnesium with excess hydrochloric acid to produce the substances shown:

Mg(s) + 2HCI(aq) -> MgClz(aq) + H2(g)

The hydrogen gas is the product that is of interest to you in this experiment. You will makean experimental determination of the number of moles of hydrogen molecules producedand the volume occupied by these molecules. The number of moles of hydrogen will be de­termined indirectly. The balanced equation for this reaction shows that the molar ratio ofmagnesium reacted to hydrogen gas produced is 1:1. Therefore, by determining the mass ofmagnesium that reacts and the number of moles that this mass is equal to, you will alsodetermine the number of moles of hydrogen gas produced. The volume of hydrogen gas

. produced will be measured directly on the scale of a gas-measuring tube. The gas laws ofBoyle and Charles will be used to correct this volume, measured under laboratory condi­tions, to the volume the sample of gas would occupy at STP. The collected data (number of

'-" moles and volume at STP) will be used to calculate the molar volume of the hydrogen gas.This experiment should aid in the understanding of the mole concept and the concept

of molar volume of a gas.

PURPOSE

Determine the volume of 1 mole of hydrogen gas at STP using experimental data,known mathematical relationships, and a balanced chemical equation.

EQUIPMENTgas-measuring tubeone-hole stopper (for gas tube)ring standutility clampthermometer

beaker, 400-mLbattery jar (or 1000-mL beaker)graduated cylinder, lO-mLmetric rulersafety glasses

MA TERIALS

magnesium ribbon (Mg) cotton thread3 M hydrochloric acid (HCI)

PROCEDURE •• ~1. Obtain a piece of magnesium ribbon from your teacher. Mea­

sure the length of the piece of ribbon (± 0.1 cm). Record thelength as (a) in your data table. Also record the mass of 1 meterof magnesium ribbon. These data will be provided by the teacher.Record as (b).

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2. Obtain a piece of cotton thread about 15 cm long. Tie one endof the thread around the piece of magnesium ribbon, leavingabout 10 cm of thread free. Bend the piece of magnesium so thatit will fit easily into the gas-measuring tube.Obtain about 10 mL of 3 M hydrochloric acid (HCI). CAU­TION: Handle this acid with care. Carefully pour the HCI intoa gas-measuring tube.

4. Tilt the gas-measuring tube slightly. Using a beaker, slowly fillthe gas-measuring tube with water at room temperature. Try toavoid mixing the acid and water as much as possible.

5. Lower the piece of magnesium ribbon 4 or 5 cm into the gas­measuring tube. Drape the thread over the edge of the tube andinsert the one-hole rubber stopper into the tube as shown in Figure19-1.

6. Add about 300 mL of water at room temperature to a 400-mLbeaker. Set up a ring stand and utility clamp, and place the beaker ofwater in the position shown in Figure 19-2.

7. Place your finger over the hole in the rubber stopper and invert thegas-measuring tube. Lower the stoppered end of the tube into thebeaker of water. Clamp the tube in place so that the stoppered end isa few centimeters above the bottom of the beaker (Figure 19-2).Record your visual observations as (g) in the data table.

8. Let the apparatus stand about five minutes after the magnesiumhas completely reacted. Then, tap the sides of the gas-measuringtube to dislodge any gas bubbles that may have become attachedto the sides of the tube. Place your finger over the hole in thestopper and transfer the tube to a battery jar filled with water.Lower the end of the tube into the water and remove your fingerfrom the hole.

9. Move the tube up or down (to equalize pressure) until the waterlevel in the tube is the same as that in the battery jar. On thescale of the gas-measuring tube, read the volume of the gases inthe tube. Record this volume as (c) in your data table.

10. In the data table, record the room temperature (d), and thebarometric pressure (e).

11. If time permits and your teacher so indicates, repeat the experi­men1.

I hole

stopper

'!I1-thread

coil of

Mg ribbon

gas tube

3MHCI

Figure 19-1

Figure 19-2

Name Date. _

EXPERIMENT 19. MOLAR VOLUME OF A GAS (CONTINUED)

'--./

OBSERV A TIONS AND DATATriall

Trial 2;A vg of 2 Trials

(a)

length of Mg ribbon ememem

(b)

mass of 1 meter of Mg ggg

(c)

volume of H2 gas in tube mLmLmL

(d)

room temperature °C°C°C

(e)

barometric pressure mmHg mmHg mmHg

(f)

water vapor pressure at mmHg mmHg mmHgroom temperature, PH20 (from Appendix B,page 204).

(g)

visual observations:

CALCULA TIONS1. Find the mass of the Mg strip:

0.050 meter _ x grams1.00 meter - mass of I-meter length

2. Calculate the number of moles of Mg reacted (which is equal tothe number of moles of H2 gas produced):

# moles Mg = mass of Mg reacted (g)24 g Mg/mole Mg

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3. Find the pressure exerted by the H2 gas in the tube:

4. Convert room temperature from DCto kelvin:K=DC+273

5. Find the volume of the H2 gas at STP:

where: PI = PH, (from calculation 3)VI = experimental value ofH2 (c)TI = room temperature (K)Tz =273 KPz =760 mm HgVz= volume of H2 at STP

6. Find the volume of 1 mole of Hz gas at STP:

moles of Hz gas (from calculation 2)volume of Hz gas (V2)

1 molexmL

x = .mL/mole= L/mole

CONCLUSIONS AND QUESTIONS1. The accepted value for the molar volume of a gas is 22.4 liters (22400 mL). How does your experi­

mentally determined value compare with this accepted value? Calculate your percentage error.2. What are some possible sources of error in this experiment?

3. Find the volume of the following masses of gases at STP:(a) 80 g O2 (b) 10 g H2 (c) 14 g N2 (d) '66 g CO2

4. How many liters would the following number of moles of any gas occupy at STP?(a) 0.25 mole (b) 0.5 mole (c) 1 mole (d) 2 moles (e) 2.5 moles

5. What happens to the other product of the reaction used in this experiment?

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