Mass of Magnesium
To determine the value of the constant of the gases, R, by
measuring the hydrogen
produced in the reaction of certain amount of magnesium with
hydrochloric acid,
according to the reaction
Mg + 2 HCl → MgCl2 + H2,
at the given conditions of pressure and temperature.
Theoretical background
The ideal gases model establishes that the relationship
between p, V, T and
amount of gas is:
P · V = n · R · T
Pressure of the gas *
Volume of the gas = number of substance * temperature* gas constant
R is a constant whose value is 0.082 atm·L/K·mol.
Materials
Support Copper wire
Clamps Magnesium
Graduated tube (20 mL) Thermometer
Hollow stopper Commercial HCl (approx. 12M)
250 mL beaker
Method/Procedure
1. Take one piece of magnesium. Record its exact weight.
2. Once clean and dry, firmly tie the magnesium tape to the
copper wire. Tie the other end of the copper wire to a hollowed stopper.
3. Pour about 2 mL of HCl conc. (≈12M) into the graduated
tube.
4. Slowly, and carefully, pour water into the tube, trying
to avoid mixing both liquids. This will be easier if you tilt the tube so it is
not vertical. As the tube fills with water you can slowly turn the tube
vertical and fill it with water up to the top.
5. Place the magnesium tape tied to the stopper onto the top
of the tube, closing the hole in the stopper with your finger, in order to
avoid any air getting in.
6. Quickly turn the tube upside down and place it in a
beaker of water. Once in the water take your finger out. You will see how the
HCl moves down the tube because it has a higher density than water. It reacts
with the magnesium, producing hydrogen gas (that will be collected at the top
of the tube).
7. Hold the tube vertically with the clamp and let it
stabilise for a few minutes to make sure that the temperature of the beaker
equals the room temperature.
8. Adjust the tube vertically until the level of the liquid
inside and outside the tube coincides so the inside pressure equals the
atmospheric pressure. The inside pressure is the sum of two factors - the
pressure corresponding to the hydrogen and the pressure corresponding to the
vapour pressure of water at that temperature:
Pin the tube = Patm =
P(H2) + PV water so then P(H2) = Patm – PV water
9. Use the above equations to calculate the pressure and
record the volume of hydrogen produced and the temperature.
10. Calculate the value of R using the equation of the
introduction, taking into account that the number of moles of hydrogen will be
the same than the ones of magnesium.
11. Repeat the whole procedure with a second piece of
magnesium and give the result as an average of the two calculations. Compare
your result with the real one.
Table 1. Water vapour pressures (1 Torr = 1 mmHg, 760 mmHg =
1 atm)
15º C 12.8 Torr 19º C 16.5 Torr 23º C 21.0 Torr 27º C 26.7 Torr
16º C 13.6 Torr 20º C 17.5 Torr 24º C 22.4 Torr 28º C 28.3 Torr
17º C 14.5 Torr 21º C 18.6 Torr 25º C 23.7 Torr 29º C 30.0 Torr
18º C 15.5 Torr 22º C 19.8 Torr 26º C 25.2 Torr 30º C 31.8 Torr
Pressure of the gas * Volume of the gas = number of
substance * temperature* gas constant
Results:
768mm and 764mm. The media is 766 mm Hg= 1.00789474 atm
V: 10,5
P:
N:1
T:20
Gas constant is 0.082 atm·L/K·mol.
Therefore: ¿What is the gas pressure?
Well, then we have to calculate it:
P*10,5 = 1*293,15*0,082
10,5 * G = 24,038
P = 2,3
The pressure of H2 is 2,3.
Moles of hydrogen will then be: 1
