Synthesis and Analysis of Zinc Chloride: Determining Composition and Formula Unit

In this experiment, the goal is to create a simple compound of zinc and chloride and determine its quantitative composition. By measuring the masses of zinc and the compound, the mass of chlorine can be derived. This information is then used to calculate the percentage composition of the product and determine the formula unit.

Apparatus:

• Hot plate
• Glass rod
• White tiles
• Measuring cylinder (10 mL)
• Analytical balance
• Crucible tongs

Chemical reagents:

• 6M HCl
• Zinc powder

Procedure:

1. Weigh the crucible and record its exact mass.
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2. Place approximately 0.25 g of zinc into the crucible, weigh it with its contents, and determine the exact mass of zinc.
3. Carefully add 10 mL of 6M HCl solution to the crucible with zinc, stirring gently with a glass rod. A chemical reaction occurs, releasing hydrogen gas. Perform this step in a fume cupboard, away from fire sources, as wet hydrogen gas can cause explosions.
4. Add 5 mL of 6M HCl incrementally until all the zinc dissolves, ensuring the total acid used does not exceed 20 mL.
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5. Place the crucible on a hot plate in the fume cupboard, slowly heating the content to avoid splattering.
6. Gently heat the compound until completely dry, ensuring it does not melt.
7. Allow the crucible to cool to room temperature and then weigh it.
8. Reheat and cool the crucible, repeating the procedure until the difference in mass does not exceed 0.02 g.
9. Determine the mass of zinc chloride from the final sample weight and calculate the mass of chlorine in zinc chloride.
10. Calculate the formula unit of zinc chloride.
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Results:

Observations:

1. Upon adding HCl to the zinc powder in the crucible, gas bubbles were released, and a grey solution formed.
2. During heating, the grey solution clarified and turned yellow.
3. Hydrogen gas was emitted during the heating process until the solution nearly dried.
4. After approximately 2 hours of heating, a white precipitate formed.

Discussion:

1. The decision not to weigh the content while hot is based on good laboratory practice. Weighing hot objects can introduce convection currents, altering the air pressure above the scale and yielding unstable readings.
2. The heating of the zinc and hydrochloric acid suspension produced a white gas, identified as hydrogen. Due to its flammable nature, the heating process occurred in a fume chamber to prevent potential explosions.
3. The reaction equation between zinc and HCl is represented as follows: Zn(s)+2HCl(l)→H2​(g)+ZnCl2​(s)
4. The mass of zinc (x g) is calculated as w−v.
5. The mass of chlorine (z g) is determined by y−w.
6. The formula unit of zinc chloride is calculated as follows: ZnCl2​

	Zinc, Zn	Chlorine, Cl
Mass	0.25g	0.25g
Number of moles	0.25g/ 65.4 g mol- = 3.823 x 10-3	0.25g / 35.45 g mol- = 7.052 x 10-3
Ratio	3.823 x 10-3 / 3.823 x 10-3 = 1	7.052 x 10-3 / 3.823 x 10-3 = 1.845
Simplest whole ratio	1	2

Through the execution of this experiment, we successfully transformed zinc powder into zinc chloride by combining it with acid and subsequently subjecting it to heat. Additionally, we determined the formula unit of zinc chloride to be ZnCl2. The reaction equation can be expressed as follows: Zn(s)+2HCl(l)→H2​(g)+ZnCl2​(s)