Measuring the Relative Atomic Mass of Oxygen and Aluminium:

Introduction

This experiment is done with the aluminium/air cell. It is necessary to have a balance that can weigh to +/- 0.01g; a mg balance would be even better.

For the measurement of oxygen the key point is that the oxygen reacts with the aluminium, and that this oxygen is "fixed" into the cell as aluminium hydroxide. (See Section 3.3 of the instructions.) The cell thus becomes heavier as it is used. The valency of oxygen is 2, and thus for every 2 electrons that flow, the mass of the cell increases by the mass of one oxygen atom.

For the measurement of aluminium the mass of dry aluminium before and after a known charge has flowed must be measured.

The experiment assumes a reasonable understanding of the chemistry outlined in section 3.3 of the instructions. The experiment could be adapted as one that verifies this chemistry, rather than one which measures something.

The measurement for oxygen can be found much more quickly than that for aluminium, and many teachers may wish to restrict the experiment to measuring the relative atomic mass of oxygen.

The apparatus needed is:-

aluminium anode

cathode

ammeter, 0-1 A

variable resistor, 0- ~2 ohms

3 connecting wires

stop clock

balance, sensitivity 0.01g or better

Instructions

1) Weigh the dry aluminium anode.

2) Make up a solution of NaCl in water, and get the cell ready as described in section 2.4 of the instruction booklet.

3) Weigh the cell with the electrolyte and cathode in it.

4) Connect the cell to an ammeter and a variable resistor, quickly set the current to a convenient current such as 0.5 amps. Start the stop clock.

5) Leave the cell running like this for as long as possible, certainly for at least 30 minutes. Adjust the resistor from time to time to keep the current constant, and give the cell an occasional gentle swirl or shake to clear the aluminium hydroxide from between the electrodes.

6) Record the stopping time, and break the circuit, so that the current stops.

7) Weigh the cell again.

8) Empty out the cell, and thoroughly rinse with water. Allow to dry. (This may have to be done overnight).

9) Weigh the dry anode again.

10) Calculate the relative atomic mass of oxygen and aluminium using the method as in the "likely results" section below.

Likely Results

Measurements:-

These results were obtained using the procedure above:-

Initial dry mass of aluminium anode = 56.583g

Initial mass of complete cell with electrolyte = 164.174g

A current of 0.5A flowed for 60 minutes.

The final mass of the complete cell = 164.310g

The final dry mass of the aluminium anode = 56.423g

Calculations:-

Using the results above, the atomic mass of oxygen and aluminium can be found.

Coulombs of charge = Current x Time in seconds = 0.5 x 60 x 60 = 1800 Coulombs

This is used to find the number of moles of electrons that flowed:-

The number of moles of electrons = 1800/96,500 = 0.0186 moles

This is then used, with the valency of oxygen, and the mass change, to find the atomic mass.

The valency of oxygen is 2, so

The change in mass of the cell = 164.310 - 164.174 = 0.136g

The results for aluminium are treated in an exactly similar way:-

The mass of aluminium reacted = 56.583 - 56.423 = 0.180g

The valency aluminium is 3, so

the quantity of aluminium reacted = moles of electrons/3 = 0.0186/3 = 0.0062 moles

These results are close to the "accepted values".