Molecularity: Atomic Weight of a Metal

• To discover the connection between electric current, electrons, and oxidation states of elements
• To apply the ideal gas law
• To determine the atomic weight of an element

Electrolysis is used for the production of several important materials on a huge scale. Aluminum, sodium, chlorine, and sodium hydroxide are produced in this way, and copper is purified electrolytically.

In this experiment you will construct an electrolysis cell that consists of a beaker containing an aqueous solution of 0.1 M acetic acid, HC₂H₃O₂, and 0.5 M sodium sulfate, Na₂SO₄, which is present to conduct electricity through the solution. Immersed in the solution are a strip of zinc metal and one end of a copper wire. The copper wire and zinc electrode are connected through a 1.5 V flashlight battery with the + terminal connected to the zinc electrode. The battery pushes electrons out of its negative terminal toward the submerged end of the copper wire, while the positive terminal of the battery pulls electrons from the zinc electrode. Zinc atoms become oxidized to Zn²⁺ at the zinc electrode, and the zinc ions dissolve into solution. The zinc electrode is called the anode, the electrode where oxidation occurs. Electrons from the battery move to the Cu wire and react with H⁺ ions in the solution, reducing them to H₂(g). Hydrogen gas rises into the closed end of the graduated cylinder, where it is trapped. The copper wire is called the cathode, the electrode where reduction occurs. The reactions occurring in the cell are thus

The volume of hydrogen gas collected will be studied as a function of the mass of Zn lost from the anode during electrolysis.

While the electrolysis is proceeding, you will carry out a second experiment, in which you will react Zn metal with an excess of strong acid, and collect the hydrogen gas produced. In this case, you will measure the volume of hydrogen gas as a function of the mass of Zn used.

1. How can data of this type be used to determine the atomic weight of Zn?
2. How do the results from the two methods compare?
3. Why must the alligator clip be kept dry when the cell is assembled for electrolysis?
4. What effect would each of the following errors have on the results of the electrolysis experiment?
   • The copper electrode was placed so that some H₂(g) did not go up the graduated cylinder.
   • The alligator clip was submerged in electrolyte.
   • The pressure of water vapor in the cylinder is ignored.
   • The Zn electrode was not dried thoroughly prior to finding the final mass.
   • The concentration of acetic acid in the electrolyte was less than 0.1M.
5. What effect would each of the following errors have on the atomic weight calculated from direct reaction data?
   • You used 9 mL of 6M HCl instead of 10.
   • The rubber stopper was not sealed well in the test tube and some gas leaked out.
   • You failed to twist the syringe plunger, so it stuck in the barrel at the 25-mL mark.

• 150-mL beaker
• 6-inch test tube
• thermometer
• test tube rack
• insulated electrical wires
• alligator clips
• electrical tape
• 10-mL graduated cylinder
• 1.5 Volt flashlight battery (size D)
• stirring rod
• 25-mL graduated cylinder (for collecting gas)
• 2-foot length of tygon tubing
• sandpaper or carborundum paper
• 50-mL glass syringe
• forceps or tongs
• #2 1-hole rubber stopper with 2-inch length of glass tubing
• clamps
• analytical balance
• glass wool
• Zn electrode
• 6 M HCl
• 0.1 M HC₂H₃O₂ in 0.5 M Na₂SO₄

Safety glasses must be worn at all times in the laboratory. You will work with solutions of hydrochloric acid, acetic acid and sodium sulfate. Avoid ingestion of solution. Hydrogen gas will be generated during the experiment. NO FLAMES PLEASE!

Record all data in your notebook.

Electrolysis.† You should set up and run two electrolyses at the same time. For each electrolysis set-up, clean the 25-mL graduated cylinder and beaker, and rinse with acetic acid/sodium sulfate solution (electrolyte). Fill the beaker with electrolyte, then fill the graduated cylinder completely from the beaker.

Carefully insert the rubber stopper into the graduated cylinder. Avoid trapping air, and obtain a good seal. Invert the cylinder into the electrolyte in the beaker and loosen and remove the rubber stopper. The cylinder must remain full. Clamp the cylinder in place so that the 25-mL mark on the cylinder corresponds with the level of electrolyte in the beaker, and rinse your hands.

Lightly sand the Zn electrode, then wipe it off with a Kimwipe. After sanding, handle the electrode with clean forceps or tongs. Find the mass of the electrode to 0.1 mg. Assemble the cell, attaching the electrical wire to the Zn electrode with an alligator clip and using electrical tape to fasten the wires to the battery. Be sure the bare end of the Cu wire is entirely within the graduated cylinder. Start the electrolysis by immersing the Zn electrode in the electrolyte so that the alligator clip does not get wet. You should see bubbles of H₂(g) on the cathode. While waiting for the two electrolyses to finish (about 2 hours), carry out the gas collection experiment.

Direct Reaction. Find the mass of a sample of zinc to 0.1 mg. Be sure that the mass lies in the range 0.100-0.120 g. Then set up the apparatus. Clamp the syringe horizontally, but do not over-tighten the clamp. BE CAREFUL NOT TO ALLOW THE PLUNGER TO FALL OUT OF THE SYRINGE. Connect the tygon tubing (B) securely to the glass tube (C), but do not connect it to the syringe yet. Rinse the test tube (E) with distilled water and dry with a paper towel. Measure 10 mL of 6M HCl and pour it carefully into the test tube. Then use a paper towel to dry the inside of the test tube above the liquid level. Insert a small wad of glass wool into the test tube and push it down using a stirring rod until it is 1/2 inch above the liquid level. DO NOT ALLOW THE GLASS WOOL TO BECOME WET! The glass wool will keep the metal separated from the acid until you initiate the reaction. Carefully add your weighed metal sample to the tube, so it rests on the glass wool. Insert rubber stopper D into the test tube and seat it firmly. Attach rubber tubing B to the tip of the syringe. Tip the test tube and agitate it gently to initiate reaction. You should see gas form as the reaction proceeds. Continue agitating until reaction is over. There should be no residual metal in the tube. Place the tube in a test tube rack and let it cool to room temperature (2-3 minutes). Lightly twist the syringe plunger to free it from sticking, then record the volume of gas in the syringe to the nearest 0.1 mL. Also record room temperature, the vapor pressure of water at this temperature, and atmospheric pressure. Dispose of materials as indicated below. Using the results from the first run, calculate the mass of Zn needed to produce 45 mL of hydrogen. Measure approximately this amount of metal and repeat the experiment. When finished, disassemble the apparatus and return it to the instructor.

Electrolysis. Remove the Zn electrode from solution when the level of liquid in the cylinder corresponds with the level of liquid in the beaker. Disconnect the dry cell. Rinse the Zn electrode (distilled H₂O), dry it completely with a paper towel, and reweigh it. Record room temperature, atmospheric pressure, and the volume of gas in the graduated cylinder. Calculate the moles of H₂(g) in the cylinder using the ideal gas law, correcting for the partial pressure of water vapor in the cylinder.

When finished, return the Zn electrodes, wires, batteries, beakers, and graduated cylinders.

The instructor will tell you how to submit your data relating volume of hydrogen collected to mass of Zn.

Direct Reaction. Remove the stopper from the test tube and pour the solution into the sink. Throw the glass wool plug in the waste basket. Flush the acid solution down the drain.

Electrolysis. Pour the electrolyte down the drain.

Name

Section

Date

1. This experiment

2. The appropriate sections of your textbook.

1. An electrolysis is carried out at 20 ^oC and an atmospheric pressure of 689 torr using the apparatus of Figure 1 with a nickel (instead of zinc) electrode. 21.4 mL of H₂(g) are collected during the electrolysis. At the same time, 0.0476 g of nickel are lost from the electrode. Calculate the atomic weight of nickel.