How to Measure Avogadro's Number in the Lab

Chemical mixtures and chemical reactions require use of a given unit known as the mole. One mole is made up of 6.022 x 10E23 subunits where E denotes an exponent. For example, one mole of water contains 6.022 x 10E23 molecules of water. The number was proposed by Italian chemist Amedeo Avogadro in 1811 and has become known as Avogadro's number. In 1827, Robert Brown provided the first experimental evidence supporting Avogadro's number using simple electrolysis. You can design a similar test to experimentally determine Avogadro's number for 1 mole of copper.

Things You'll Need

• DC power supply
• Connecting wires with alligator clips
• Zinc metal strip
• Copper metal strip
• Analytical balance accurate to 0.0001 g
• Steel wool
• 1 M sulfuric acid
• 250 ml beaker
• Laboratory or masking tape
• Goggle
• Latex or nitrile gloves
• Calculator
• 70 percent Isopropyl alcohol
• Timer

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Instructions

Electrolysis

• 1

  Clean the copper strip and the zinc strip to remove oil residue and oxides using steel wool. Metal often develops a thin layer of oxide covering the outer surface, and skin oils could inhibit electrolysis. Once the metal is clean, it can be wiped with alcohol and allowed to dry. The metal strips should not have contact with skin after it has been cleaned.

• 2

  Determine the gram weight of the copper strip. The weight of the copper strip should be recorded and will be used to calculate copper atoms transferred during electrolysis.

• 3

  Pour 200 ml of sulfuric acid into the 250 ml beaker. Sulfuric acid is extremely corrosive and caution should be used.

• 4

  Attach zinc and copper strips to connecting wires using the alligator clips. Both metal strips are then placed in the sulfuric acid and secured with tape. The metal strips should not have contact with each other or the sides of the beaker.

• 5

  Attach connecting wires to the DC power supply. The wire attached to the zinc strip is connected to the cathode (-) terminal and the wire attached to the copper strip is attached to the anode (+) terminal.

• 6

  Begin electrolysis by setting the power supply to 0.5 Amps. It is important to begin recording the time electrolysis begins and when it ends. You can record data for any period between five and 10 minutes.

• 7

  Remove the copper strip after the power supply has been turned off. The copper strip should be allowed to air-dry completely. Towel drying the strip would alter the weight and should not be done. After the copper strip is completely dry, then weigh and record the gram weight.

Calculating Avogadro's Number

• 8

  Determine the number of Coulombs during electrolysis. This is determined by converting Amps into Coulombs using the formula Amps x Time (seconds). The value determined in this experiment using 0.5 Amps for 10 minutes is 0.5 Amp x 6,000 s equals 3,000 Coulombs.

• 9

  Calculate the number of copper electrons transferred during electrolysis. Electrons are calculated by dividing Coulombs by 1.602 x 10E-19, the charge for a single electron. Therefore 3,000 Coulombs/ 1.602x10E-19 equals 1.873 x 10E22. Because each copper atom transfers two electrons during electrolysis, this would be the equivalent to 9.365 x 10E21 copper atoms, one half the number of electrons.

• 10

  Calculate the number of moles of copper metal lost from the copper strip during electrolysis. This is determined from the change in weight measured before and after electrolysis divided by the atomic weight (63.546 g per mole) of copper. For example 0.0105g copper would calculate to 1.011g/ 63.546 g per mole or 1.591 x 10E-2 mole of copper transferred during electrolysis.

• 11

  Determine the experimental value for Avogadro's number by dividing the atoms of copper by the moles of copper. The value for the given example would be 9.365 x 10E21 / 1.591 x 10E-2 and be equal to 5.886 x 10E23.