Name: _Petya Pisan___
THE MOLAR VOLUME OF HYDROGEN
Introduction:
In this experiment, you will be conducting a reaction of magnesium metal with excess hydrochloric acid in a eudiometer (gas measuring tube). The volume of hydrogen will be equalized to the room pressure and the final volume recorded.
Since the hydrogen gas will be collected over water, the hydrogen will be mixed with water vapour. The pressure of the dry hydrogen gas can be calculated by subtracting the water vapour pressure from the room pressure. Water vapour pressures at various temperatures are in the table below:
Table of water vapour pressure°C / kPa / °C / kPa
15 / 1.71 / 23 / 2.81
16 / 1.82 / 24 / 2.98
17 / 1.94 / 25 / 3.17
18 / 2.06 / 26 / 3.36
19 / 2.20 / 27 / 3.57
20 / 2.34 / 28 / 3.78
21 / 2.49 / 29 / 4.00
22 / 2.64 / 30 / 4.24
With the volume, pressure and temperature of dry hydrogen gas, you can calculate the volume at standard temperature and pressure (STP), 100 kPa and 273.15 K. The mass of magnesium used can be converted to moles of magnesium used. The volume of dry hydrogen at STP produced by one mole of magnesium can then be calculated. This is also the molar volume of hydrogen since 1 mol of Mg produces 1 mol H2.
Suppose 0.0326 g of magnesium produces 35.6 mL of hydrogen measured over water at 100.0 kPa. The room temperature is 26.0°C (299K). The temperature of the water is 19.0°C. From the table, we obtain a water vapour pressure of 2.20 kPa at 19.0 °C. The water vapour pressure is subtracted from the room pressure, using Dalton’s Law of Partial Pressure, to obtain the pressure of the hydrogen gas alone:
100.0 kPa – 2.20 kPa = 97.8 kPa
The volume of hydrogen is corrected to STP:
Thus, 0.00134 mol Mg (0.0326 g produced 0.0314 L H2 or 31.4 mL H2 measured at STP.
Purpose: To determine the volume of hydrogen gas produced when a given quantity of magnesium reacts with excess hydrochloric acid.
Pre-Lab: /7 K/U /10 I /4C
1) What is present in the eudiometer (gas collection tube) above the liquid phase, once the reaction in this experiment is complete? Explain your answer. (2 K/U)
The by-product of the reaction of dilute hydrochloric acid and magnesium creates hydrogen gas and water vapour in the eudiometer.
2) What is the reaction that is taking place in this experiment? Indicate all reactants and products with subscripts. (2 I)
This is a mono displacement reaction where Mg gets oxidised and HCl gets reduced forming MgCl2 and H2 as by-product.
Mg (s) + 2HCl (aq) ---> MgCl2 (aq) + H2 (g)
3) If the room temperature is 23.5 °C, and the atmospheric pressure is 99.5 kPa, what mass of magnesium would be used to produce 75.0 mL of hydrogen gas? Show all calculations clearly.
(4 K/U)
Please see scanned sheets for calculations
Given
Room temperature = 23.5 Celsius + 273K = 296.5K
4) What is the volume of water vapour if 65.0 mL of hydrogen gas are produced? Explain your answer. (2 I)
5) Explain what equalizing the pressure of the gas in the eudiometer means. (I K/U)
Equalizing the pressure of the gas means to provide a volume of the gas under standard pressure and temperatures (100KPa & 273.15K) or a volume in equalization with the pressure and temperature of the classroom.
6) Why must the pressure in the eudiometer be equalized after the reaction is complete? Explain your answer. (2 I)
Eudiometer pressures must be equalized due to the effect of the water vapour pressure exerted on the hydrogen gas which will cause a false volume on the hydrogen if not corrected.
7) Why is copper wire a good choice for the cage to hold magnesium? (1 I)
Copper wire is a good choice for holding magnesium because it doesn’t react with the hydrochloric acid.
8) What would the volume of hydrogen be from the electrolysis of 0.0600 mol of water? Show all of your work. (3 I)
See scanned sheets
9) Prepare a flowchart of the procedure and a data table for the experiment (4 C)
Data Table 1.
Item / ResultsAtmospheric pressure
Mass of Mg metal
Volume of H2 gas
Room temperature
Water temperature
Water vapour pressure
Materials:
· Eudiometer
· Copper wire
· Ring stand
· Buret clamp
· 600 mL beaker
· 100 mL graduated cylinder
· One-hole rubber stopper (size 00)
· Battery jar
· Thermometer
· Barometer
· Magnesium ribbon
· 6 M HCl
Procedure:
SAFETY: Wear eye protection and latex gloves throughout. Ensure that there are no open flames or sources of ignition as the hydrogen gas produced in this experiment is highly flammable.
1) Obtain between 0.07 and 0.08g of magnesium ribbon. Record the mass in your data table.
2) Roll the piece of magnesium into a coil. Use fine copper wire to build a copper cage around the magnesium. The cage should allow the acid to enter but it should not have large openings though which small pieces of magnesium might fall. The copper cage must be small enough to fit into the gas measuring tube. Leave a long piece of extra copper wire attached to the cage to act as a handle.
3) Set up a ring stand and buret clamp, and fill a 600 mL beaker about two-thirds full of water.
4) Carefully pour 10 mL of 6 M HCl into the eudiometer. Slowly fill the tube completely with tap water. The 6M acid is denser than water and so it will remain at the bottom of the eudiometer. The tube should be completely willed with water so that when a one-hoe stopper is placed in the tube a little water will be forced out. Any bubbles sticking to the walls of the tube may be dislodged by tapping the tube gently.
5) Holding the wire handle, lower the copper cage containing the magnesium into the water to a depth of about 5 cm. Place the one-hole stopper into the mouth of the tube so that the wire handle is caught between the glass and the stopper (not in the hole of the stopper).
6) Cover the hole in the stopper with your finger and invert the tube. Place the inverted tube in the beaker of water. Remove your finger, and clamp the tube in position using the buret clamp. The hydrochloric acid will eventually reach the magnesium ribbon and a reaction will occur. There is no danger that the acid will reach the stopper before you remove your finger.
7) When the magnesium ribbon has disappeared and the reaction has finished, wait for 2 min to allow the gas in the tube to come to room temperature. Dislodge any gas bubbles that are sticking to the walls of the tube by gently tapping the tube.
8) Cover the hole in the stopper with your finger and transfer the tube to a battery jar or a large graduated cylinder filled with water. Remove your finger and adjust the tube so that the level of the water inside the tube is the same as the level of the water outside the tube. When the two levels are equal, the pressure on the water outside the tube (room pressure) equals the pressure on the water inside the tube caused by the gases in the tube. Read the gas volume. Drain and clean the gas measuring tube. Record the volume in your data table
9) Measure the room temperature and the temperature of the water in the battery jar. Use this latter temperature to find the water vapour pressure. Read the room pressure from the classroom barometer. Record the temperature and pressure in your data table.
Analysis: see scanned sheets
1) Calculate the number of moles of magnesium used. Show all calculations. (2 A)
2) Calculate the pressure exerted by the dry hydrogen gas. Show all calculations. (1 A)
3) Calculate the volume that the dry hydrogen would have occupied at STP. Show all calculations.
(2 A)
4) What is the volume of 1 mol of hydrogen (the molar volume) at STP as determined by this experiment? (2 A)
5) Calculate the percent difference between your answer and the accepted value for the molar volume of hydrogen gas at STP. (2 A)
Marking Scheme for Lab:
10 marks Communication based on communication rubric and proper lab format
18 marks Application (2 data table, 9 analysis, 2 error analysis, 2 conclusion, 3 lab performance)