Name________________________________________________________________ Date________________ Period___
The Molar Relationship Between Mass and Volume
Purpose and Introduction:
Both solids and gases must often be handled in the same experiment. Amounts of solids
are easily determined by using a balance. But you can’t use a balance to find the mass of a gas. Instead, the chemist usually
measures the gas volume, using this value to calculate the mass of the gas. In order to do this, it is necessary to know the
quantitative relationship between mass and molar volume. AVOGADRO’S LAW explains the relationship between molar
volume, molar mass and the actual mass of a sample of gas. This law states that the volume of a gas depends on the number
of particles of the gas, and at STP, one mole of any gas occupies 22.42L. Also, equal volumes of gases at the same
temperature and pressure contain equal numbers of particles. One can set up Avogadro’s Law as a ratio, and solve for an
unknown quantity:
V=(a)(n) ;
V1 = V 2
n1
n2
In this experiment you will investigate the significance of Avogadro’s Law. You will determine the volume of hydrogen
gas evolved from a reaction between Mg metal and hydrochloric acid. You will collect this gas over water. From your
results, you will compare the volume of the dry hydrogen gas produced, changed to STP conditions, with the theoretical
molar volume of 22.4 L.
Materials and Methods:
Materials:
Normal lab equipment and Mg ribbon, eudiometer, pressure equalization apparatus, copper wire cage, 3M
HCl, plastic cup, thermometer, barometer, analytical balance.
Methods:
1) Obtain a plastic cup ¾ full of tap water.
2) Prepare a data table. Record the ambient air pressure and temperature in the lab.
3) Cut a piece of thick Mg ribbon about, but close to, 3.5cm long (7.1 for 100ml). Polish off the oxidation with steel wool.
Take the mass of your Mg piece to  0.0000g on an analytical balance. It is not necessary to use a weigh boat this time.
4) Obtain a copper wire cage affixed to a one- or two-holed stopper that will fit in your eudiometer. This will hold the Mg
piece where it will react best with the HCl. Fold up the Mg ribbon and place it in the cage. Make sure it is secure.
5) Set up a ring stand with a buret clamp like the model on the prep table.
6) Slowly pour about 10ml of 3M HCl into the 50 ml eudiometer (19 – 21ml for 100ml).
7) Incline the eudiometer and slowly trickle in some deionized water from the wash bottle. After carefully layering a
cushion of water on top of the acid, carefully pour in more water until the eudiometer is completely full.
8) Affix the Mg ribbon-in-a-cage to the eudiometer snugly and top off with more deionized water.
9) With your index finger over the holes of the stopper, slowly invert the eudiometer and place the stoppered end in the
plastic cup containing the reservoir water. Clamp the eudiometer in place such that the bottom of the stopper is slightly
above the bottom of the plastic cup. The reaction will not start immediately. Be patient.
10) Toward the end of the reaction, what is left of the Mg may escape the cage and float out. This is normal and okay. Let
the reaction finish. When the evolution of gas has completely stopped, spank the eudiometer at the stoppered end with a
wooden pencil to dislodge any gas bubbles stuck to the walls of the eudiometer or the stopper.
11) Place your index finger over the holes in the stopper and remove your eudiometer, carrying it over to the pressureequalization apparatus. Lower your eudiometer into the apparatus until the level of the water in the eudiometer and the level
of water in the apparatus are exactly the same. Record the volume of gas in the eudiometer. This will make the pressure of
your gas mixture (“total pressure”) the same as ambient atmospheric pressure.
12) Empty the contents of the eudiometer and wash with a little soap and the buret brush. Rinse well with tap water (shake
test), then rinse with deionized water three times. Dry with acetone and an air wand.
Data/ Results/ Calculations:
1) Do enough stoik to calculate the moles of H2 that should be theoretically produced in your reaction from your Mg mass.
2) Using a gas law and your calibration curve, calculate the partial pressure of your hydrogen gas from your experiment.
3) Using a gas law, convert the actual volume of your hydrogen gas to an STP volume. Calc your actual mol of H2.
4) Using a gas law, set up a ratio. Think about this: a fractional part of a mole of hydrogen gas at STP should produce some
fractional amount of the standard molar volume, 22.42L. Use this ratio to calculate the volume of hydrogen gas produced in
your experiment, corrected for STP conditions, extrapolated to molar volume. The easiest way to do this is to use
Avogadro’s Law, so set up a ratio: “1.00 mol H2 is to 22.42L as ____mol H2 (from your data) is to ____L. Or, use some
other method of your own choosing. Also, show calcs for % yield: [(actual H2 mol / theoretical H2 mol (from step 1)] x100.
Conclusions/ Discussion:
Two questions are written on the back; include with your lab report. Write your report (show all calcs) on the back.
Make sure you thoroughly rinse (after washing) your eudiometer. Rinse it finally with dH2O. Clean up your lab area.