Forces and Bonding: Gases

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

• To determine the molar mass of a gas from measured mass and volume
• To explore the relationship between volume and temperature for a number of gases
• To explore the relationship between volume and pressure for a number of gases
• To explore the relationship between volume and amount for a number of gases
• To determine whether the identity of the gas influences these relationships

The gas phase was the last of the three common phases of matter to be appreciated, because most gases are invisible and cannot be sensed by touch. It was not until the mid-18th century that techniques for handling and measuring gases were developed. The state of a gas is characterized by citing the temperature (T) of the sample, the pressure (P) exerted on (or by) it, the volume (V) it occupies, and the amount (n) of it that is present. The relationship among these four quantities turns out to be quite simple for gases. In this experiment, we will explore several aspects of this relationship.

Focus Questions

Please keep the following questions in mind as you perform the experiment.

1. In Part 1, why is it important to withdraw the plunger to the 50-mL mark before weighing the empty syringe (i.e., why can't the syringe be weighed with the plunger all the way in)?
2. For Part 1, construct a working curve using your data for the three known gases (i.e., plot mass of gas or density of gas versus molar mass for the three known gases).
3. Use your working curve to determine the molar masses of the two unknown gases.
4. From Part 2, what is the relationship between gas volume and celsius temperature at fixed pressure and amount of gas? (HINT: construct an appropriate plot.)
5. Can you use your data from Part 2 to estimate absolute zero in celsius degrees? (HINT: use your plot.)
6. From Part 3, what is the relationship between gas volume and applied pressure at fixed temperature and amount of gas? (HINT: construct an appropriate plot.)
7. From Part 4, what is the relationship between gas volume and amount of gas at fixed applied pressure and temperature? (HINT: construct an appropriate plot.)
8. Does either of the relationships from parts 2 and 3 change when a different amount of gas is used? If so, how?
9. Does any relationship change when a different gas is used? If so, how?

Equipment and Materials

• 50- or 60-mL plastic syringe with hole thru barrel and 2 holes thru flange at top of barrel
• nail to place thru plunger hole
• 50-mL glass syringe
• small septum caps
• known gases: N₂, He, Ar
• unknown gases
• analytical balance
• 1000-mL beakers
• 100-mL beakers
• Bunsen burner
• ring stand/ring/wire gauze
• bathroom scale
• gas generator (250-mL sidearm flask, length of rubber tubing, rubber stopper)
• water-displacement gas collector (25-mL graduate, 250-mL beaker)
• small graduated cylinders

Safety glasses must be worn at all times in the laboratory. You will work with hot water and open flames. Please be careful.

Experimental

Record all data in your notebook. Obtain the required equipment.

Part 1: Gas Density Measurements Determine the mass of the empty 50-mL (60 mL) plastic syringe. To do this, push the plunger completely in; place the small septum cap over the syringe tip; draw the plunger out until the hole in the plunger is exposed; insert the nail in the hole; and weigh the syringe.You will determine the mass of a 50-mL (60-mL) sample of each of three known gases. You should reweigh the empty syringe at the end of the experiment.

Fill the syringe with helium by drawing the gas into the syringe until the plunger hole is just exposed. Insert the nail, and push in the plunger until the nail encounters the top of the syringe barrel. Then place the septum cap on the tip of the syringe. Weigh the assembly. Empty the syringe into the fume hood. Repeat once more for helium, then do duplicate runs for nitrogen and argon.

Fill the syringe with the first unknown gas, and weigh. Repeat once more, then do duplicate runs for the second unknown gas.

Empty the syringe by ejecting the gas in the fume hood.

Part 2: Relationship of Volume to Temperature Prepare a water bath by filling the 1000-mL beaker with water. Set up a ring stand fixed with a wire gauze/ring. BE SURE THE RING STAND IS UNABLE TO PIVOT! Place the water bath on the wire gauze/ring. Place a thermometer in the water. Connect the Bunsen burner to the gas jet and light it, but do not heat the water at this point. Prepare a second bath by filling a 1000-mL beaker half full with ice, and adding water to the 800 mL mark.

Examine the glass syringe to be sure that it has been properly lubricated with oil by the instructor. The plunger should slide smoothly and easily in the barrel, and a continuous film of lubricant oil should be visible. If you are uncertain about the condition of the syringe, please show it to the instructor before proceeding. Fully insert the plunger, then seal the syringe tip using a small septum cap. With instructor assistance, inject about 30 mL of one of the available gases into the syringe via the septum cap. Note which gas you use.

Place the syringe tip-down in the ice bath and stir the bath. After about 5 minutes, read the bath temperature and the volume of gas in the syringe.

Mount the syringe in the water bath using a clamp to support the flange of the syringe barrel. Let the syringe come to thermal equilibrium with the bath, then read and record the bath temperature and the volume of gas in the syringe. Place the burner under the bath and heat with stirring until the temperature has been increased by about 10 ^oC. Remove the burner, allow the temperature to stabilize, and again record T and gas volume. Continue to measure gas volume at intervals of 10 ^oC until you reach 90 ^oC. After recording your final temperature and volume, remove the syringe from the bath and allow it to cool to room temperature.

Repeat the series of measurements a second time using a different volume of the same gas.

Empty the syringe by ejecting the gas in the fume hood.

Part 3: Relationship of Volume to Pressure Using a ruler, carefully measure the inside diameter of the barrel of the 60-mL plastic syringe. Fill the plastic syringe to capacity (50 or 60 mL) with one of the known gases and cap off the syringe with a septum cap. Read and record the volume of gas in the syringe. Place the end of the syringe plunger flush against the surface of a bathroom scale. Using your left hand to hold the septum cap on the end of the syringe, use your right hand to push the syringe against the scale until the volume of the gas has decreased by 10 mL from the initial value. Record the gas volume and the "weight" registered by the scale. Continue in this manner in 10-mL volume increments until the gas volume has been decreased to about 1/4 of its initial value. This should give you about 5 pressure-volume data points. The instructor will provide you with the value of atmospheric pressure. When analyzing your pressure-volume data, please remember that

• the pressure of gas in the syringe is the sum of atmospheric pressure and the additional applied pressure;
• the additional applied pressure must be calculated as the force registered by the bathroom scale divided by the inside cross-sectional area of the syringe barrel.

Empty the syringe by ejecting the gas in the fume hood.

Part 4: Relationship of Volume to Amount (Mass)

Weigh an empty 50-mL plastic syringe on the analytical balance using the method of Part 1. Fill the syringe to the 10-mL mark with one of the known gases, seal the syringe with a septum cap, pull the plunger out to 60 mL, insert the nail stop, and reweigh. Record gas volume and total mass. Remove the nail, add 10-mL more of gas, and reweigh by the same procedure. Continue adding 10-mL increments of gas and weighing until the syringe contains about 60 mL of gas when at atmospheric pressure.

Empty the syringe by ejecting the gas in the fume hood.

Part 5: Laboratory generation of Common Gases

You should perform this part of the experiment only if told to do so by your instructor.

Generation of CO₂ by Chemical Reaction. Assemble a gas-generation apparatus by attaching a length of rubber tubing to the sidearm of a sidearm flask. The flask should be equipped with a tight-fitting rubber stopper. Place about 1 gram of powdered calcium carbonate in the sidearm flask, quickly add 10 mL of 6M HCl, and quickly stopper the flask. Wait about 10 seconds so that air is flushed from the flask, then place the free end of the rubber tubing into a 250-mL beaker. Allow the beaker to "fill" with carbon dioxide.

When the beaker is full, "pour" its contents onto the flame of a Bunsen burner. What happens?

Generation of H₂ by Chemical Reaction. Clean and dry the gas generation apparatus. Place about 1 gram of zinc metal into the flask. Quickly add 10 mL of 6 M HCl, and quickly stopper the flask. Wait about 10 seconds so that air is flushed from the flask, then place the free end of the rubber tubing under the mouth of a 25-mL graduated cylinder that is filled with water and inverted in a pan of water. The hydrogen gas will displace the water from the graduate. Collect 10-15 mL of hydrogen gas in the graduate, then let the reaction finish in the fume hood. Can you think of a way to demonstrate that the collected gas is hydrogen?

Generation of Cl₂ by Chemical Reaction. Clean and dry the gas generation apparatus. Place about 1 gram of potassium permanganate into the flask. Quickly add 10 mL of 6 M HCl, and quickly stopper the flask. Wait about 10 seconds for air to be flushed from the flask, then place the free end of the rubber tubing under the mouth of a 25-mL graduated cylinder that is full of water and inverted in a pan of water. Chlorine gas will displace the water from the graduate. Collect 10-15 mL of chlorine gas in the graduate, then let the reaction finish in the fume hood. Can you think of a way to demonstrate that the collected gas is chlorine? (HINT: chlorine is a strong bleaching agent.)

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

• Clean glassware by recommended procedures.
• Disassemble and clean the glass syringe.
• Disassemble and clean the gas generator.
• Return all borrowed equipment to the instructor before leaving lab.

Empty syringes of gas by ejecting in a fume hood.

Preparation
Forces and Bonding: Gases