Discusion for Experiment 1

Chemistry
EXPERIMENT 1
Determina7on Of Formula Unit Of A Compound
By : Kharolrena E Wong


• To determine the formula unit of zinc chloride, apply the technique of MaMoRa!
!
• MaMoRa stands for Mass, Mole and Ratio!
!
• Therefore, in this experiment, we need to find the mass of zinc and chlorine.!
Experiment 1
Determina7on Of The Formula Unit Of A Compound


Example :! !
!
Mass of empty crucible :
50.6074 g
Mass of crucible + zinc powder :
50.8600 g
Mass of zinc powder :
0.2526 g
Mass of crucible + zinc chloride:
First hea7ng
51.134 g
Second hea7ng
51.124 g
Third hea7ng
-
Mass of zinc chloride:
51.1240 g – 50.6074 g = 0.5166 g (actual mass)
Mass of Chlorine :
0.5166 g – 0.2526 g = 0.2640 g
Experiment 1
Determina7on Of The Formula Unit Of A Compound


Let’s do the MaMoRa
Zn
Cl
Mass (g)
0.2526
0.2640
Mole (mol)
0.2526 g / 65.4 gmol-1 = 3.8623 x 10-3
0.2640 g / 35.5 gmol-1 = 7.4366 x 10-3
Ra7o
3.8623 x 10-3 3.8623 x 10-3
=1
7.4366 x 10-3 3.8623 x 10-3
= 1.93
=2
Therefore, the empirical formula
for zinc chloride is :
Experiment 1
ZnCl2!
Determina7on Of The Formula Unit Of A Compound


• Theoretically, the formula unit for zinc chloride is ZnCl2.!
!
• The calculated results shows that this experiment get close to the theoretical value.!
!
Experiment 1
Determina7on Of The Formula Unit Of A Compound


• However, some might get different results.
• Itmaybe: – Cl < 2
– Cl > 2
!
How could this happen?!
Experiment 1
Determina7on Of The Formula Unit Of A Compound


Cl < 2
Cl > 2
The big reason for this is because the mass of zinc powder. It must be less than it should be.
• Perhapsthezincpowderis not pure. (it might has dust and you weighed together the dust, hmmmm.
• Perhaps,youdidnot dissolved the zinc powder fully.
The reason for this is more to the mass of zinc chloride.
• Zincchlorideisa hygroscopic compound. It tends to absorb moisture from surrounding. When this happen, it will affect the reading of the mass of zinc chloride. You’ll get bigger reading than it is suppose to be (and the mass of chlorine will be bigger too)
* hcps://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/z/Zinc_chloride.htm
Experiment 1
Determina7on Of The Formula Unit Of A Compound


To overcome the errors, here are some suggestions :!
• HCl should be added slowly – to avoid formings of bubbles!
• Heat slowly – to avoid splattering!
• Slow cooling process – to avoid absorption
of moisture.!
• Do not weigh while it is still hot – stable reading will not be achieved.!
Experiment 1
Determina7on Of The Formula Unit Of A Compound


Let’s relate your experiment’s data with what you learn in chapter 1 :
In this experiment, you have 0.25 g on zinc powder and 10 ml of 6M HCl.
i. Determine the limi7ng reactant
ii. Calculate the percentage yield of the product.
Experiment 1
Determina7on Of The Formula Unit Of A Compound


Determine the limi7ng reactant
Step 1
Zn(s) + 2HCl(aq) ! ZnCl2(s) + 2H2(g)
Step
2 Find the given mol
mol of Zn
= 0.25g/65.4gmol-1 = 3.8623 x 10-3
mol of HCl
= (6.0 M)(0.01 L) = 0.06 mol
Step
3 Find the stoichio metry
According to the equa?on :
1 mol Zn react with 2 mol HCl
3.8623 x 10-3 mol Zn react with 7.7246 x 10-3 mol HCl
Therefore :
Step
4 Compare and conclude
Since number of mole HCl needed is less than the number of mole given, therefore we can conclude that HCl is excess reactant.
Therefore, Zinc is the limi?ng reactant
Experiment 1
Determina7on Of The Formula Unit Of A Compound


Calculate the percentage yield of the product
Step 1
According to the equa?on :
1 mol Zn produce 1 mol ZnCl2
Therefore :
3.8623 x 10-3 mol Zn should produce 3.8623 x 10-3 mol ZnCl2
This is theore7cal mol
Step
2 Find the mass of TY
Mass of theore?cal yield =
3.8623 x 10-3 mol x 136.4 g mol-1 = 0.5268 g
theore7cal mass
Step
3 Calculate the PY
% yield = =
=
actual yield x 100% theore?cal yield
X 100 %
0.5166 g 0.5268 g
98.06 %
Experiment 1
Determina7on Of The Formula Unit Of A Compound