Forces and Bonding: Molar Volume of a Gas

1 lab period; work in pairs. Complete the Preparation page before laboratory.

• To learn gas collection techniques
• To estimate the value of the ideal gas constant, R
• To discover the volume occupied by a mole of a typical gas under specified conditions of temperature and pressure

The gaseous phase of matter was the last to be recognized by early scientists because most gases are invisible, and all gases require techniques of collection and handling that are not necessary with the liquid and solid phases of matter. In this experiment, hydrogen gas will be generated via reaction (1) and will be collected by displacement of water from a 10-mL graduated cylinder. This technique, the first discovered for gas collection, was developed in the 18th century. The setup shown here will be used.

Focus Questions

1. How many moles of magnesium were used in each of your 2 runs?
2. How many moles of hydrogen gas were produced in each run?
3. Calculate the partial pressure of hydrogen gas in the cylinder.
4. Estimate the value of the ideal gas constant, R.
5. Estimate the molar volume of hydrogen gas.
6. Estimate the precision in the molar volume based on your 2 runs.
7. Can you suggest improvements to the experiment that would enable you to determine the molar volume more precisely?

Equipment and Materials

• Labkit
• Mg ribbon
• Concentrated HCl (12 M)

Safety

Safety glasses must be worn at all times in the laboratory. Concentrated HCl is corrosive. Avoid contact with skin or clothing. In the event of contact with skin, flush with copious amounts of cold water for at least 5 minutes. The gas generated in this experiment is flammable. No open flames please.

Experimental

Record all data in your notebook. Weigh a piece of magnesium ribbon having a mass no more than 0.009 g and no less than 0.006 g.

Fill your 10-mL graduate with distilled water. Using a graduated pipet and syringe pipet pump (Labkit), measure 2-mL of concentrated HCl. Insert the pipet slowly into the graduate until the pipet tip is near the bottom of the graduate, then slowly and carefully empty the pipet. The objective here is have the concentrated HCl stay at the bottom of the graduate. It will naturally tend to do this, given careful enough dispensation, because it is more dense than water. Wrap your small piece of Mg ribbon in a small piece of Kimwipe (4 x 4 cm), roll the Kimwipe into a wad, and insert the wad into the top of the graduate. Then plug the top of the graduate with a rubber stopper, and quickly invert the graduate in a 100-mL beaker filled with distilled water. Remove the rubber stopper, and observe subsequent events.

When the process is finished, equalize the water levels inside and outside of the graduate, and determine the volume of collected gas. Measure the laboratory temperature and pressure, and correct the pressure for the vapor pressure of water at lab temperature.

Repeat the gas collection experiment an additional time.

Clean-up. When you have finished all of your work:

• Clean all glassware with Alconox and water and shake dry.
• Rinse Pasteur pipets by inserting the nozzle of your wash bottle in the top end and squirting water through. Aspirate dry.
• Return all components to the labkit.
• Return all borrowed equipment to the instructor before leaving lab.

Spent reaction solutions containing dilute HCl and MgCl₂ may be flushed down the drain with plenty of water.

1. This experiment.
2. The appropriate or assigned sections of your textbook.