Determination of the Thickness of Zinc on a Piece of
Galvanized Metal
Purpose:
In this experiment, you will determine the thickness of zinc on a piece of galvanized steel
gravimetrically.
Overview:
Relying on the differences in activity between zinc (the galvanizing coating) and the
coated steel, the zinc will selectively be removed using hydrochloric acid and the mass of
the zinc coating determined. The thickness of the coating will be determined using this
mass, the coated area of the metal and the density of zinc.
Active metals such as zinc react with acids to release hydrogen gas according to the
following equation:
2HCl (aq) + Zn (s) → H2 (g) + Zn2+ (aq) + 2Cl- (aq)
Iron is much less reactive than zinc and so it reacts with acid only slowly. It is therefore
possible to use the rate of bubble formation to signal when removal of the zinc coating is
complete.
Materials:
Chemicals:
6 M HCl
3 to 5-cm rectangular piece of galvanized metal
Equipment:
centimeter ruler
250-mL beaker
balance
Hazards:
Concentrated solutions of HCl will burn skin and damage clothing. Fumes from acid can
be corrosive and/or irritating. Do this experiment in a well-ventilated area. Goggles
must be worn throughout this experiment.
Procedure:
1. Determine the mass of the metal and record.
2. Measure the dimensions of the piece of metal to determine its surface area.
3. Place the metal in the beaker and cover with the 6 M HCl solution. When the gas
bubbles stop appearing rapidly, pour the acid and metal piece into a sink rinsing
thoroughly with water. Dry the metal that remains.
4. Determine the mass of the remaining metal and record.
5. Calculate the surface area of the metal.
6. Given that the density of zinc is 7.14 g/cm3, calculate the volume of zinc on the
metal.
7. Determine the thickness of the zinc coating.
8. Given that the radius of the Zn atom is 2.66 × 10-8 cm, calculate the thickness of
the zinc coating in terms of zinc atoms.