Department of Chemistry
University of Missouri-St. Louis
Molar Volume of Hydrogen - Microscale
Name ___________________
Purpose: to use a single displacement reaction to produce hydrogen and then to calculate its
molar volume at STP.
Reference: Microscale Chemistry, Alan Slater and Geoff Rayner-Canham, Addison- Wesley
Background: All gas samples containing the same number of molecules will occupy the same
volume under the same conditions of temperature and pressure. This is Avogadro's hypothesis.
A molar volume of a gas is the volume occupied by one mole of a gas at STP, or one atmosphere
and 0 C. The reaction studied in this laboratory is magnesium metal reacting with excess
hydrochloric acid. Mg(s) + 2 HCl (aq)  MgCl2 (aq) + H2 (g) Look at the mole ratio of the
reactants to the products. If one reacts one mole of magnesium metal, one would expect one
mole of hydrogen gas to be produced. You will be given a small amount of magnesium metal
and asked to find the moles of magnesium reacting and therefore the moles of hydrogen gas
expected. You will use the gas laws to help you calculate the volume of hydrogen gas produced
under laboratory conditions. From this information, you can calculate the molar volume of
hydrogen gas at STP.
Materials:
1 50 mL beaker
1 hand lens
1 metric ruler
1.0 M HCl
1 10 mL syringe
1 barometer
1 clean Mg ribbon piece (The instructor
will inform you of the mass of a known
length of ribbon.)
Procedure:
1. Measure as accurately as possible a small piece of magnesium ribbon (use a magnifier!).
2. Remove the plunger of the syringe. Place the magnesium ribbon into the barrel of the syringe.
Let it drop to the bottom. Replace the plunger.
3. Draw up about 2 ml of distilled water into the syringe. Holding it upright, gently expel the
water so that the tip of the syringe will still contain water but the barrel has no more water.
4. Fill the syringe with 10 mL of 1.0 M HCl by drawing up the acid from a beaker. Do NOT
allow any bubbles other than the bubbling of the magnesium ribbon reacting with the acid.
Gas bubbles will form and force the acid solution out of the hole in the syringe. The beaker will
catch the drips.
5. Once all the Mg ribbon has reacted, wait a few minutes for the contents of the syringe to cool
to room temperature. Record the final volume of the liquid in the syringe.
6. Record the barometric pressure in mm Hg and the room temperature.
7. Return the acid solution to the waste container provided.
8. Record the mass of one meter of Mg ribbon. Your teacher will have this information.
9. Find the water vapor pressure at room temperature from a table of vapor pressures. Record.
Data Table:
Length of Magnesium Ribbon Cut
Mass of 1.0 Meter of Mg Ribbon
Volume of HCl in Syringe at Start
Volume of HCl in Syringe at End
Volume of Hydrogen Gas in Syringe
Room Temperature
Barometric Pressure
Water Vapor Pressure at Room
Temperature
Calculations: SHOW YOUR WORK.
1. Find the mass of the magnesium piece that you used.
2. Calculate the number of moles of Mg reacted from the grams of magnesium used. This
equals the same number of moles of hydrogen gas in the balanced equation.
3. The total pressure in the syringe is the sum of the hydrogen gas and the water vapor
pressures. Use Dalton's Law of partial pressures to find the pressure exerted by the hydrogen
gas.
4. Convert room temperature to Kelvin.
5. Find the volume of hydrogen gas at STP from the volume you measured in the syringe.
6. Find the volume of ONE mole of hydrogen gas at STP using your data.
7. The accepted value for the molar volume of a gas at STP is 22.4 liters. Calculate your
percentage error.
8. What are some possible sources of error in this experiment?