Pahang Matriculation College
Chemistry Unit
Session 2021/2022
CHEMISTRY LAB REPORT SK015
Practicum Class
Experiment : 1
11DSH45
Determination Of The Formula Unit Of A Compound.
Objectives
1. Synthesize a zinc chloride compound.
2. Determine the formula unit of zinc chloride.
Introduction
One of the main properties of a compound is its chemical composition which can
be identified by determining the elements present. A quantitative analysis can be
used to determine the composition of an unknown compound. Once the
composition of the compound is known, it’s formula unit can be determined. For
example, a compound containing 0.1 mole of silver and 0.1 mole of bromine will
have a formula unit, AgBr.
In this experiment, a simple compound composed of zinc and chlorine will be
prepared. Once the mass of zinc and the mass of the compound are known, the
mass of chlorine can be determined. Using these masses, the percentage
composition of the compound can be calculated and the formula unit can be
deduced.
Procedure (passive
sentences)
1. The crucible is weighed and the exact mass is recorded.
2. 0.25g of zinc powder is placed into the crucible and the exact mass of zinc
powder is determined.
3. 10ML of 6 M HCl solution is carefully added into the crucible containing the
zinc powder and it is stirred gently with a glass rod. A vigorous chemical
reaction occurred and hydrogen gas is released.
4. If the zinc powder did not dissolve completely, the acid is added, 5 mL at a
time until all zinc is dissolved. The amount of acid used did not exceed 20mL.
5. The crucible is placed on a hot plate in the fume cupboard and the content is
heated slowly so that the compound does not splatter during the heating
process.
6. The compound is heated gently until it is completely dry. The crucible is
removed from the hot plate immediately to avoid the compound from
melting.
7. The crucible is covered and it is allowed to cool to room temperature. Then,
the crucible and the compound is weighed. The mass is recorded.
8. The crucible is reheated to dry the compound. It is let to be cooled to room
temperature and then, weighed again. The procedure is repeated until the
difference in mass does not exceed 0.02g.
9. The mass of zinc chloride is determined from the final weight of the sample
(the smallest value). The mass of chlorine in the zinc chloride is calculated.
10. The formula unit of zinc chloride is determined.
Result
1. i. Mass of empty crucible
ii. Mass of crucible + Zinc powder
iii. Mass of Zinc powder
= 54.0328 g
= 54.2850 g
= 0.2522 g
2. Mass of crucible + Zinc Chloride for;
i.First heating
ii.Second heating
iii.Third heating (Mass of final weight)
= 54.5531 g
= 54.5431 g
= 54.5431 g
3. i. Mass of Zinc Chloride
ii.Mass of Chlorine
Discussion
-Calculation (if any)
-Graph (if any)
-compare result with
theory
- precaution
-error
=54.5431 g–54.0328 g
= 0.5103 g
= 0.5103 g – 0.2522 g
= 0.2581 g
(a) Calculation of the formula unit:
Zn
Cl
Mass (g)
0.2522 g
0.2581 g
No. of mole
0.2522 = 0.0039 mol 0.2581 = 0.0073 mol
(mol)
65.4
35.5
Simplest ratio
0.0039 = 1
0.0073 = 2
0.0039
0.0039
Formula unit of zinc chloride is ZnCl2 ( theory: ZnCl2 )
(b) Precautions:
1. Wear gloves while pouring HCl into the crucible to avoid contact with the
skin.
2. Carry out the mixing of the chemical reagents and the heating process in
a fume cupboard, and avoid working near fire source. This is because wet
hydrogen gas released during chemical reaction can cause explosions.
3. Heat the content in the crucible slowly in the fume cupboard to avoid the
compound from splattering during the heating process, which can cause
injury to the student.
Conclusion
Answer of exercises
1.
Zinc chloride compound can be synthesized by mixing zinc powder and
hydrochloric acid.
2.
Formula unit of zinc chloride is ZnCl2 ( theory : ZnCl2 )
1. The content is not weighed while it is still hot because the reaction is still
occurring.
2. Crucible needs to be covered during cooling because the impurities from
air can combine with the conpound and affect the mass reading.
3.
Zn(s) + 2HCl(aq) → ZnCl2 (aq) + H2 (g)
Prepared by: Mdm Norhasni J