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MOLAR VOLUME OF A GAS LAB

DISCUSSION: Avogadros hypothesis states that equal volumes of all gases contain equal numbers of

molecules under the same conditions of temperature and pressure. It follows from this hypothesis that all

gas samples containing the same number of molecules will occupy the same volume under the same

conditions of temperature and pressure. A special name is given to the volume occupied by 1 molesamples of gases at STP. This volume is called MOLAR VOLUME.

The basis of this experiment is the following reaction in which you will react a known mass of magnesiumwith excess hydrochloric acid to produce the substances shown:

Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g)

The hydrogen gas is the product that is of interest to you in this experiment. You will make an

experimental determination of the number of moles of hydrogen molecules produced from the magnesium

and the volume occupied by these molecules. The number of moles of hydrogen will be determinedindirectly. The balanced chemical equation for this reaction show that the molar ratio of magnesium

reacted to hydrogen gas produced is 1 : 1; therefore, by determining the mass of magnesium that reacts

and the number of moles that this mass is equal to, you will also determine the number of moles ofhydrogen gas produced. The volume of hydrogen gas produced will be measured directly on the scale of a

gas measuring tube. The combined gas law will be used to correct this volume, measured under lab

conditions, to the volume the sample of gas would occupy at STP. The collected data (number of molesand volume at STP) will be used to calculate the molar volume of the hydrogen gas.

PURPOSE: Determine the volume of 1 mole of hydrogen gas at STP using experimental data, knownmathematical relationships and the concept of molar volume of a gas.

EQUIPMENT:

Gas measuring tube 400 mL beakerOne-hole stopper (for gas tube) battery jar or 200 mL graduated cylinder in a central locationRing stand graduated cylinder, 10 mL

Utility clamp

WEAR SAFETY GOGGLES AND APRONS

MATERIALS:

Magnesium ribbon strip (Mg) 3 M hydrochloric acid (HCl) thread

PROCEDURE:

1. Obtain a piece of magnesium ribbon about one cm in length. Measure its length in millimeters.

2. Obtain a piece of cotton thread about 15 cm long. Form the Mg into a circle, tie it with the thread,

then wrap the thread around the Mg many times to make a cage. Leave enough thread free to

drape over the edge of the gas measuring tube to hold the Mg in place.

3. Place 10 mL of 3 M HCl in the gas measuring tube. HANDLE THIS ACID WITH CARE!

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4. Tilt the gas measuring tube slightly. Using a beaker, slowly fill the tube with dH2O. Try to avoid

mixing the acid and the water as much as possible.

5. Put about 300 mL of tap water into the beaker and place it on the ring stand. Place a utility clampon the ring stand pole to hold the gas measuring tube.

6. Lower the piece of Mg ribbon 4 or 5 cm into the gas measuring tube. Drape the thread over the edgeof the tube and insert the one-hole stopper into the tube. (Some water should get forced out.)

7. Place your finger over the hole in the rubber stopper and invert the gas measuring tube. Lower thestoppered end of the tube into the beaker of water. Clamp the tube in place so that the stoppered

end is just above the bottom of the beaker.

8. Record your visual observations.

9. Let the apparatus stand about five minutes after the magnesium has completely reacted. Tap the sides

of the tube to dislodge any gas bubbles that may have become attached to the sides of the tube.Place your finger over the hole in the stopper and transfer the tube to a battery jar (or 2000 mL

graduated cylinder) filled with water. Lower the end of the tube into the water and remove

your finger from the hole.

10. Move the tube up or down (to equalize the pressure) until the water level in the tube is the same as that

in the graduated cylinder. On the scale of the gas measuring tube, read the volume of the gases inthe tube. Record this volume in your data table.

11. If you have a small piece of magnesium remaining, find the volume of the gas quickly, take the piece

of magnesium out of the tube. Dry it off and find its mass. Calculate the mass of magnesium that

did react.

12. Record the room temperature and barometric pressure in the data table.

Make a DATA TABLE including the following information:

Mass of magnesiumVolume of hydrogen gas in the tube

Room temperature

Barometric pressure (mmHg)Water vapor pressure at room temperature

CALCULATIONS:

1. Calculate the number of moles of Mg reacted. Grams Mg / Atomic Mass Mg

mole Mg = mol H2

2. Find the pressure exerted by the hydrogen gas in the tube.

PH2 = barometric pressure water vapor pressure

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3. Convert room temperature from celsius to Kelvin.

4. Find the volume of the hydrogen gas at STP

P1V1 = P2V2 Solve combined gas law for V2

T1 T2P1= pressure exerted by the hydrogen gas (#2)

V1= volume of gas measured

T1= room temperature (#3)T2= 273K

P2= 760 mmHg

V2= ? (volume of H2gas at STP)

5. Find the molar volume.

Volume of H2gas at STP (#4) = mL/mole convert to L/mole (divide by 1000)Moles of H2gas (#1)

CONCLUSION:State your molar volume and support it with evidence.

Five sentence paragraph minimum.

D.O.E:Calculate % errorWhat are sources of error?

How does you value compare to the accepted value?

D.O.T.:What is the big picture?