SUBJECT CODE
: SK0013 CHEMISTRY
PRACTICAL NO.
: 02
TITLE
: DETERMINATION OF THE FORMULA UNIT OF A COMPOUND
GROUP MEMBERS :
Aisyah Firzanah Binti Abd Malik
(FS11110019) (Group 1)
Nursakinah
Najwah
Binti
Suhaimi (FS11110003) Muhammad
Aiman
Bin
Osman
(FS11110111) Meidin Bin Mantedan (FS11110012) DATE OF PRACTICAL
: 20/06/2011
DEMONSTRATOR
: Encik Mohd. Hafiz Bin Abd. Maid
EXPERIMENT 2: DETERMINATION OF THE FORMULA UNIT OF A COMPOUND
Objectives 1. To synthesise zinc chloride compound.
2. To determine the formula unit of zinc chloride.
Introduction One of the main properties of a compound is its chemical composition which can be identified simply by determining the elements present. Although qualitative analysis is very useful, chemists also need to know the quantitative aspect of the compound. A quantitative analysis can be used to determine
the
composition
of
an
unknown
compound.
Once
the
composition of the compound is known, its 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 of AgBr. In this experiment, you will prepare a simple compound of zinc and chloride. 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 product can be determined and the formula unit can be calculated.
Apparatus Hot plate, glass rod, white tiles, measuring cylinder (10mL), analytical balance, crucible tongs.
Chemical reagents 6M HCl, Zinc powder. Procedure 1. The crucible is weighed and the exact mass is recorded. 2. Approximately 0.25 g of zinc placed into the crucible. The crucible
weighed with its contents and the exact mass of zinc is determined. 3. 10 mL of 6M HCl solution is added carefully into the crucible
containing the zinc powder and the contents is gently stirred using a glass rod. A vigorous chemical reaction occurred and hydrogen gas was released. Precaution: This step is carried out in a fume cupboard. A fire source is kept away because wet hydrogen gas can cause explosions. 4. When the zinc powder has not dissolved completely, 5 mL of 6M HCl
is added at a time until all of the zinc is dissolved. The amount of acid used cannot exceed 20 mL. 5. The crucible placed on a hot plate in the fume cupboard and the content slowly heated so that the compound does not splatter during the heating process. 6. The compound is gently heated until it is completely dry and is
ensured not to melt. 7. The crucible is allowed to cool to room temperature then it is
weighed. 8. The crucible is reheated and kept to cool in room temperature then it
is weighed again. This procedure is repeated until the difference in mass does not exceed 0.02 g.
9. The mass of zinc chloride is determined from the final weight of the
sample and the mass of chlorine in the zinc chloride is calculated. 10.
The formula unit of zinc chloride is calculated.
Results: Mass of crucible, v (g) Crucible + content (zinc powder), w (g)
25.6 25.85
Mass of zinc, x (g)
0.25
Mass of zinc chloride, y (g)
26.1
Mass of chlorine, z (g)
0.25
Observations: 1. When HCl is poured into the crucible content zinc powder, bubble gas
was released and grey solution was formed. 2. While heated, the grey solution become clear and then turned yellow. 3. Hydrogen gas was released while the heating process (until solution
is almost dry). 4. After heated for about 2 hours, white precipitate was formed.
Discussion: 1. The reason why the content is not weighed while it is still hot because
it is always poor laboratory procedure to weigh something when it is still hot because the hot object will radiate heat in the form of convection currents in the air above the scale. This will alter the air pressure above the object unpredictably and produce unstable readings of the object being massed. 2. When the suspension of zinc and hydrochloric acid was heated, a
white gas was released. This gas is hydrogen gas. The heating process was done in a fume chamber because hydrogen gas is flammable and may cause an explosion. 3. Based on the result, the equation for the reaction between zinc and
Hal is : Zn(s) + 2HCl(l) H2(g) + ZnCl2(s) 4. Mass of zinc, x (g) = w – v 5. Mass of chlorine, z (g) = y – w
6. To calculate formula unit of zinc chloride:
Mass Number of moles Ratio
Simplest whole ratio
Zinc, Zn
Chlorine, Cl
0.25g
0.25g
0.25g/ 65.4 g mol-
0.25g / 35.45 g mol-
= 3.823 x 10-3
= 7.052 x 10-3
3.823 x 10-3 / 3.823 x 7.052 x 10-3 / 3.823 x 1010-3
3
=1
=
1
1.845 2
So, the formula unit of zinc chloride is ZnCl2
Conclusion: By conducting this experiment, we were able to synthesise zinc powder to become zinc chloride by mixing it with acid and then heating it. Furthermore, we were also able to determine the formula unit of zinc chloride, which is ZnCl2. The equation of the reaction: Zn(s) + 2HCl(l) H2(g) + ZnCl2(s)
