Lab experiment for calculating Magnesium mass

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