Preliminary experiment -01
Gas law: The quantitative displacement of hydrogen by metals
THEORY
When an acid is reacted with a solid metal, a reduction-oxidation (redox) reaction occurs to
form hydrogen gas and metallic cations,
2 H+ (aq) + Mg (s)
H2 (g) + Mg2+ (aq)
Here, the hydrogen has been reduced while the magnesium metal has been oxidised. The
relative atomic masses of a metal, can be determined from a knowledge of the amount
(number of moles) of metal reacted. This experiment therefore depends on the measure of
the amount (number of moles) of hydrogen gas product. The volume and the total pressure
of the collected gas are to be measured in Parts A and C. However, the collected gas is a
mixture of
• air from the vial
• water vapour
• hydrogen from the reaction.
The contribution from the “air” and the water vapour will be determined separately. In Part
B, you will measure the effect of injecting 1 mL of liquid into the reaction vial. This forces
humid air into the measuring cylinder. Hence the true volume of humid hydrogen from the
reaction is found by
V (measured) = V (humid air from injection) + V (humid H2 from reaction)
V (humid H2 from reaction) = Vhydrogen
In Part D, you will determine how the humidity inside the measuring cylinder affects the
measured pressure. The total pressure within the container is equal to the atmospheric
pressure when the water levels inside and outside the container are equal.
Ptotal = Pvapour + Phydrogen
Another way to think of this is
P (measured) = P (water vapour) + P (dry hydrogen)
P (dry hydrogen) = Phydrogen
The (water) vapour partial pressure, Pvapour, depends only on the temperature and can be
subtracted from the total pressure to give the hydrogen partial pressure, Phydrogen. This
hydrogen pressure is then related to the number of moles by the deal gas law.
Phydrogen Vhydrogen = n RT
The amount (number of moles) of metal is related to the mass, m, and relative atomic mass,
Mr.
n = m/Mr
Phydrogen Vhydrogen = m/Mr RT
Procedure — Part A: Mg + acid reaction
(1) Set up the apparatus as shown above adding 0.004 g – 0.006 g of Mg to the reaction
vial. Make sure that the measuring cylinder is completely filled with water.
(2) Draw 1.0 mL of 5 M HCl into the syringe. Push the needle through the septum and
inject the concentrated acid into the vial.
The reaction of acid with magnesium is very fast.
(3) Once the reaction has ceased raise or lower the measuring cylinder so that the water
levels inside the cylinder is the same as the overall water level in the beaker outside
the cylinder.
Part B: Measuring the injection volume
(1) Remove the measuring cylinder and re-set up the apparatus. You do not need to
empty the vial.
(2) Draw up 1.0 mL of water into the syringe inject the water into the vial.
(3) Determine the volume of humid air pushed from the vial through the pipe into the
measuring cylinder by the injection.
Part C: Zn + acid reaction
(1) Disassemble the experimental apparatus and thoroughly rinse the reaction vial with
(deionised) water.
(2) Add 0.012–0.017 g of Zn into the reaction vial. Reassemble the experimental
apparatus, making sure that the measuring cylinder is completely filled with water.
(3) Make sure that there is no water inside the syringe, then carefully draw up 1.0 mL of
5M HCl into the syringe and inject the concentrated acid into the vial.
The reaction of Zn + acid is very slow.
Part D: Atmospheric pressure and temperature
(1) Use the Fortin barometer to measure the atmospheric pressure by measuring the
height (in mm) of the mercury column. 760.000 mm Hg = 101.325 kPa = 101 325 Pa.
(2) Measure the water temperature. This will be used in calculations involving the ideal
gas law.
Results
Briefly summarize the results of your measurements.
Metal 1
Metal 2
Name and symbol of metal
Mass of empty weighing boat / g
Mass of metal plus weighing boat / g
Mass of metal / g
Temperature of water / K
Atmospheric pressure / mm Hg
Atmospheric pressure / Pa
Vapour pressure of H2O / Pa
Pressure of H2 / Pa
Volume of H2 collected plus injection volume/mL
Injection volume / mL (Part B)
Volume of H2 collected / mL
Volume of H2 collected / m3
Water vapour correction
• Using the table, estimate the water vapour partial pressure at the measured temperature.
A temperature of 17.3°C has a water vapour pressure of 1.97 kPa. (see the chart from your
TA)
Calculation of the relative atomic mass
(1) Calculate the volume of humid hydrogen produced by the each reaction.
(2) Calculate the partial pressure of (dry) hydrogen.
(3) Determine the amount (number of moles) of (dry) hydrogen produced by the
reactions.
(4) Using the balanced molecular equation for the reactions calculate the amount
(number of moles) of metal reacted.
(5) Substituting your values for amount (number of moles) and the mass of metal
reacted, determine the relative atomic mass of Mg and Zn
(6) Check your values with the “accepted” values from the textbook.