Molar Volume of Hydrogen Gas
Introduction
How would you measure the mass and volume of a gas? This is actually quite a
difficult problem because gases often seem to lack definitive properties. If you filled a
balloon with hydrogen gas and then tried to measure the combined mass of the hydrogen
and the balloon, you would obtain an inaccurate measurement because of the buoyancy of
the surrounding air. You could try to measure the volume of the hydrogen, but gas
volume changes with variations in temperature or pressure. If you placed hydrogen gas
in a rigid container, it would expand to fill the entire volume of the container regardless
of the amount of hydrogen present. Also, gases can be produced or absorbed by chemical
and physical changes of solids or liquids, as in some decompositions or in evaporation
and condensation.
The mystery of gas properties was a major stumbling block to the development of
modern chemistry. A solution to this mystery was found when scientists discovered that
the changes in gas volume caused by pressure and temperature variations could be
mathematically predicted. Another pivotal discovery was the realization that the mass of
a gas can be determined by measuring the change in mass of the solid or liquid that
produced the gas.
In 1811, Avogadro published his hypothesis that equal volumes of different gases
at a given temperature and pressure contain identical numbers of gas particles. This
hypothesis led to the idea that a mole of a given gas (6.02 x 1023 particles) occupies the
same volume as a mole of any other gas, as long as the temperature and pressure are the
same.
In this investigation you will determine and experimental value for the molar
volume of hydrogen gas. You also will see if your results confirm the accepted molar
volume of 22.4 liters per mole at a temperature of 0C and a pressure of 760 mm Hg (one
atmosphere). To make your results comparable, you will keep the gas as close as
possible to 0 C and do a calculation to adjust for differences in pressure.
Pre Lab Questions:
1. Why is this investigation carried out in a beaker of ice water that is at or near 0 C?
2. Write the balanced equation for the reaction that occurs between magnesium and
hydrochloric acid.
3. How is the volume of gas collected at room temperature adjusted to standard
pressure?
4. What happens to the acid that is placed in the graduated cylinder at the start of the
experiment? How might this pose a safety hazard?
Safety:
Wear goggles at all times. If you spill any hydrochloric acid, neutralize the spill with
baking soda and clean up completely.
Procedure:
1. Put on your goggles. Fill a 1000 ml beaker 1/3 full of ice. Add tap water until is
nearly full. Place a thermometer in the beaker. Add more ice if necessary until
the temperature is 0 C. Read the barometer to measure the atmospheric pressure
in the laboratory. Record the temperature and pressure in the Data Table.
2. Obtain a strip of magnesium that is about 0.5 cm long. With scissors, trim the cut
edges so that they are straight across. Measure the length of the strip to the
nearest 0.02 cm and record the length in the Data Table.
3. Use a dropper to place approximately 3 ml of 3.0 M HCl into the bottom of an
empty 10ml graduated cylinder. Try to avoid getting acid on the sides of the
cylinder.
4. Rinse out the dropper. Fill the rest of the graduated cylinder with tap water using
the micropipette. Fill the rest of the graduated cylinder with tap water using the
micropipette. Drip the water down the inside surface of the cylinder to prevent
mixing the acid with the water.
5. Place the magnesium carefully onto the surface of the water in the cylinder. It
should float. Quickly cover the cylinder with a square of plastic wrap, stretch the
plastic tight, and secure with a rubber band. Make sure that there are no air
bubbles trapped under the plastic wrap.
6. With a pen or pencil, poke a small hole in the plastic over the cylinder mouth.
Holding the cylinder by the base (to avoid heating it), immediately invert it in the
beaker of ice water. Hold the cylinder vertically with its mouth submerged. Do
not block the hole.
7. The magnesium will react with the acid, producing hydrogen gas that collects in
the cylinder. When the reaction is complete, chill the gas by submerging the
cylinder completely in the ice water for about one minute, tipping it at at a slight
angle if necessary.
8. To read the volume of hydrogen gas in the cylinder, lift the cylinder vertically
until the liquid level inside the cylinder matches the level of the water in the
beaker. This equalizes the gas pressure in the cylinder with the atmospheric
pressure outside the cylinder. Record the volume of hydrogen gas in the Data
Table.
9. If time permits, carry out a second trial by repeating steps 2 – 8.
10. When you are finished, rinse the diluted acid in the beaker down the drain. Avoid
getting the acid on yourself. Clean up your work area and wash your hands.
Data Table
Trial 1
Trial 2
Length of Mg
strip (cm)
Volume of
hydrogen
collected (ml)
Temperature of
water in beaker
( C)
Atmospheric
Pressure (mm Hg)
Calculations
1. Find the mass of magnesium used in each trial, using the following formula and
the mass-to-length ratio provided by your teacher
Mass (g) = mass/length ratio x Mg length
2. Use the equation you wrote in Question 2 of the pre-lab questions to identify the
number of moles of hydrogen produced per mole of magnesium. Then calculate
the number of moles of hydrogen produced during the lab.
3. Find the volume of hydrogen at standard pressure used in each trial, using the
following formula:
Volume = measured volume (in liters) x atmospheric pressure (mm Hg)
760 mm Hg
Questions:
1. Based on your data, calculate the molar volume of hydrogen (liters H2/moles
H2) and compare to the accepted value of 22.4 L/mole by computing the percent
error.
2. How would you explain the percent error for your calculation of molar volume?
(sources of error)
3. How would you redesign the experiment to reduce the percent error?
4. Why do you think the graduated cylinder was inverted in the ice water bath
rather than being held right side up?
5. What type of chemical reaction (single replacement, double replacement,
decomposition, or synthesis) occurred between the magnesium and hydrochloric
acid?