DETERMINATION OF THE THICKNESS OF ZINC ON A PIECE
OF GALVANIZED METAL
In this experiment, students will determine the thickness
of zinc on a piece of galvanized Iron. They will be introduced
to the reactivates of metals with acids and will develop some
feeling for the size of an atom.
This experiment is appropriate for a general or first-year college-prep
course. Knowing the density of zinc, students can determine the
thickness of the zinc coating on the metal by determining the
area of the metal covered with zinc and the mass of zinc removed
when the metal is reacted with hydrochloric acid. If students
are also given the radius of a zinc atom, they can calculate the
thickness of the coating in terms of number of zinc atoms.
Approximately 25 minutes.
Concentrated solutions of HCl will burn skin and damage clothing.
Fumes from acid can be caustic and/or irritating; do this experiment
In a well-ventilated area (hood, if available). Goggles must be
worn throughout this experiment.
- 6 M HCI (500 mL concentrated HCl solution diluted to 1.00 liter
with distilled or deionized water)*
- 3 to 5-cm rectangular piece
of galvanized metal *
- centimeter ruler
- 250-mL beaker*
- centigram balance
- HCl solution is available from a hardware store as muriatic
acid, 28% HCl; this solution is approximately 8 M and may be substituted
for 6 M HCl solution.
- Scraps of galvanized metal are available from contractors or
a piece of galvanized metal can be purchased from a hardware store.
- A Styrofoam cup or small glass jar may be substituted for the
Diluted acid solution may be flushed down the drain with water.
Remaining metal may be disposed of with solid waste.
Active metals such as zinc react with acids to release hydrogen
gas according to the following equation:
- Determine the mass of the metal and record.
- Use the ruler to measure the length and width of the metal
- Place the metal in the beaker and cover with 6 M HCl solution.
When the gas bubbles stop appearing rapidly, add water; pour off
the diluted acid, and rinse and dry the metal that remains.
- Determine the mass of the remaining metal and record.
- Calculate the area of the metal rectangle covered by the zinc.
Remember the zinc covers two surfaces of the metal.
- Given that the density of zinc is 7.14 g/cm3, calculate the
volume of zinc on the metal.
- Determine the thickness of the zinc coating.
- 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.
2 HCl (aq) + Zn(s)----> H2 (g) + Zn2+(aq) + 2 Cl-(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.
Have students write down the formulas for the area and volume
of the zinc coating and the density of the zinc. By manipulating
the equations, students can solve for the thickness of the zinc
* Remind students that the coating covers both sides of
the galvanized metal.
Toon and Ellis, Laboratory Experiments for Foundations of
Chemistry ; Holt / Rinehart & Winston, 1973. A similar
experiment is described in this work.
Submitted by Annis Hapkiewicz
Woodrow Wilson Leadership Program in Chemistry
The Woodrow Wilson National Fellowship Foundation
CN 5281, Princeton NJ 08543-5281