The Molar Volume of a Gas

    You will determine the standard molar volume of hydrogen gas in this experiment.  You will begin by reacting a known mass of magnesium metal with an excess of hydrochloric acid.  The hydrogen gas produced will be collected by displacing water in a gas collection tube.  But whenever a gas is collected over water, the result is a mixture of the collected gas and water vapor.  Dalton’s law of partial pressures states that the total pressure of any gas mixture is equal to the sum of the component pressures of each of the gases.
    Ptotal = Px + Py + Pz + … for any number of gases.

  In this experiment, this statement becomes
    Ptotal = Phydrogen + Pwater vapor.

The gas collection tube can be adjusted so that the pressure of the gas inside the tube is the same as the air pressure outside, that is Ptotal = Proom.  The vapor pressure of water can be obtained from a reference table.

    Having determined the volume and the pressure of the hydrogen at the temperature of the lab, you can use the combined gas laws to find the volume this sample of gas would occupy at STP.  The number of moles of hydrogen can be determined from the balances chemical equation and the mass of magnesium used.

1)    Observe the reaction of magnesium and hydrochloric acid.
2)    Apply Dalton’s law to find the partial pressure of collected hydrogen gas.
3)    Determine the volume this gas would occupy at STP.
4)    Calculate the volume of one mole of this gas at STP.

Barometer ( for class)    
Beral pipet
Ring stand
Utility clamp
Gas collection tube
400-mL beaker
2-hole rubber stopper to fit gas collection tube
500-mL or 1000-mL graduated cylinder,
battery jar or large plastic bucket
metric ruler
analytical balance (optional)
safety goggles
plastic gloves
laboratory apron

magnesium ribbon
copper wire
3 M hydrochloric acid, HCl

Prelab Questions
1)    Write a balanced equation  for the reaction of magnesium and hydrochloric acid.
2)    What is the ratio of moles of magnesium used to moles of hydrogen produced in the above reaction?
3)    What is meant by STP?
4)    What two gases will be collected in the gas collection tube?
5)    What is the function of the copper wire in this experiment.
6)    What piece of information from a reference table will you need in order to complete the calculations in this experiment.
7)    A volume of a 35.0 mL of nitrogen gas was collected over water at 24 C and 99.1 kPa.  The vapor pressure of water at this temperature is 2.99 kPa.
a.    What is the pressure of the nitrogen gas alone?
b.    What would the volume of this gas be at STP?

1) Put on your safety goggles, plastic gloves, and laboratory apron.
2)    Record the barometric pressure on the Report Sheet.
3)    Obtain a piece of magnesium ribbon approximately 4 to 5 cm long.  If an analytical balance is available, measure the mass of the ribbon directly to the nearest 0.01g and record the mass on the Report Sheet.
4)    Obtain a piece of fine copper wire approximately 15 cm long.  Roll the magnesium into a loose ball and encase it in a “cage” constructed from the copper wire .   Be sure to leave several centimeters of the copper wire free to use as a “handle”.
5)    Assemble a ring stand and clamp for supporting the gas collection tube as shown in Figure A.
6)    Add approximately 300 mL of room temperature tap water to a 400- ml beaker.
7)    Fill the collection tube to the brim with water.  If there are any bubbles of air in the tube, gently tap the side of the tube to dislodge the bubbles, then add more water so that the tube is filled to the brim with water.
8)    Put your finger over the open end of the tube, invert the tube, and submerge this end in the water in the large beaker.  Secure the apparatus with a  clamp as shown in Figure A.  Inspect the tube to be sure there are no bubbles of air inside it.  If there are bubbles, repeat steps 7 and 8.
9)    Hold the copper wire handle and insert the copper cage, holding the magnesium up into the gas collection tube.  Keep the end of the handle outside of the tube.  Secure the wire handle with the one-hole stopper as shown in Figure B.
10)    Fill the beral pipet nearly full with 3 M hydrochloric acid.  Make sure there are no bubbles in the stem of the pipet and only one small bubbles in the bulb of the pipet.  Immerse the end of the pipet under the water in the large beaker.  Keeping this end under the water, bend the stem of the pipet upward and stick it into the hole in the one-hole stopper.
11)    Squeeze the bulb of the pipet slowly to force the hydrochloric acid into the bottom of the collection tube.  Do not allow the bubble of air in the bulb  of the pipet to enter the collection tube.  Pull the stem of the pipet gently out of the hole in the stopper while you are still squeezing the bulb.  Observe the magnesium in the wire cage.  If the bubbling stops before all the magnesium has reacted, add more 3 M hydrochloric acid by repeating steps 10 and 11.
12)     After all the magnesium has reacted, allow about 5 minutes for the solution to cool to room temperature.  Find the temperature of the water/ acid mixture remaining in the beaker.  Record it on the Report Sheet.
13)    With your gloves still on, cover the stopper hole with your finger and transfer the tube to the large graduated cylinder filled with room temperature water.  Raise or lower the tube until the level outside the tube as shown in Figure C.  Record the volume of gas collected to the nearest 0.1 mL.
14)    Remove the copper “cage” and place it in the container designated by your teacher.  Pour the water/acid mixture from your collection tube and the large beaker into the waste acid container designated by your teacher.  Rinse all glassware with water.
15) Before leaving the lab, clean up all materials and wash your hands thoroughly.

The Molar Volume of a Gas
Data and Calculations

Data Table 1
Room pressure from barometer                     
Temperature in the beaker
Volume of gas collected

For each of the calculations, show your work in the space provided, then record your results in the Data Table.
1)    What is the mass of the magnesium strip that you used?
2)    How many moles of magnesium does this represent?
3)    How many moles of hydrogen are produced in this reaction?
4)    What is the vapor pressure of water under the conditions of this experiment?
5)    What is the pressure of just the hydrogen gas?
6)    What would be the volume of this gas at STP?
7)    What is the volume of one mole of this gas at STP?

Data Table 2
Mass of magnesium strip                       
Moles of magnesium used
Moles of hydrogen produced
Vapor pressure of water
Pressure of hydrogen gas
Volume of hydrogen at STP
Volume of one mole of hydrogen at STP

Analysis and Conclusions
1)    How would the volume of hydrogen collected differ if 10 cm of magnesium ribbon had been used?
2)    The accepted value for the volume of 1 mole of any gas at STP is 22,400 mL per mole.  What is the percent error of your experimental value?
3)    What are some possible sources of error in this experiment?
4)    How would changing the concentration of the HCl used affect the results of this experiment?
1)    How does Avogadro’s hypothesis relate to the results of this experiment?
2)    Using what you have learned n this experiment, explain why a balloon filled with helium will float up, but an identical balloon dilled to the same point with air at the same temperature and pressure will not float up.