THE MOLAR VOLUME OF HYDROGEN:
THE REACTION OF MAGNESIUM WITH HYDROCHLORIC ACID
• The molar volume of hydrogen gas will be determined by experimentally measuring the volume of hydrogen evolved during the reaction of magnesium with hydrochloric acid.
• The universal gas constant will be measured and class data will be combined for statistical data analysis. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Laws and concepts used:
• Mol amount, mol = mass of compound (g) / molecular mass (g/mol)
• Theoretical moles of H2:
1 mole H 2 mol Mg ×
1 mole Mg
• Total atmospheric pressure will be determined by subtracting the temperature correction factor from the barometer reading, in torr (1 torr = 1 atm = 760 mm Hg)
• The water vapor pressure should be taken in account by first measuring the water level in the eudometer above the beaker water level and converting it into mm of mercury (use conversion factor for density of water over density of mercury):
• hHg, Equivalent height of mercury (torr)=hH2O × 1.00/13.59
• For the partial pressure of hydrogen apply Dalton’s law of partial pressures Ptotal = Ppartial
• The volume of hydrogen gas should be determined by using the combined gas law considering the standard conditions: Pstp = 760 torr = 1 atm, Tstp = 273K
• The universal gas constant will then be determined as
R = STP STP nTSTP EXPERIMENTAL PROCEDURE:
1. Weigh on an analytical balance a precut magnesium ribbon: record the weight to the fourth significant figure after the decimal point (please make sure the mass is around 0.08 g).
2. Compress the weighed sample into a tight bundle.
3. Cut about 20 cm strip of copper wire and wrap the bundle in all directions with the copper wire to form a little cage (leave about 5 cm of the wire straight for a handle). The cage will hold the metal and prevent it from sticking to the walls of the eudiometer and also speed up
the reaction by providing a concentrated ‘electron bed’ for the reduction of hydrogen ions on the copper surface).
4. Slide the straight end of the copper wire into the hole of a rubber stopper and bend it to make a hook. The magnesium bundle should extend above the edge of the stopper about 2.5 inches. 5. Obtain a clean 100 mL eudometer (long narrow tube with one side closed). The reading of the scale of the eudiometer is from the top to the bottom, just like in a buret.
6. Slowly add about 6 mL of concentrated hydrochloric acid (Caution: conc. HCl solution is highly corrosive). Slowly and very carefully add disilled water to the eudiometer: use the squize bottle and direct the water flow to the wall of the eudiometer. You should see a distinct layer of acid below water.
7. Cover the eudiometer with the rubber stopper with metal sample, and add more water through the hole of the stopper to completely fill the tube.
8. Cover the hole with your finger and quickly invert the eudiometer into a 400 mL beaker partially filled with water. Secure the eudometer and observe concentrated acid slowly descending down to the metal.
9. As the metal reacts with the acid, hydrogen gas will replace the water in the tube.
10. Wait until there is no more hydrogen bubbles observed.
11. Tap the eudometer to free any hydrogen bubbles adhering to the sides of the tube or the copper wire.
12. Take the temperature of the gas by holding the thermometer in contact with the side of the eudiometer. 13. Measure the volume of gas evolved: it should be equal to the volume of water replaced in the eudometer (record the volume with appropriate precision).
14. Measure the difference between the water levels in two vessels by using a metric ruler to calculate the equivalent pressure in mm of