final formal report
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Experiment Number:
3
Experiment:
Gravimetric Analysis of a Chloride Salt
DATE:
26 Feb. 2014
Name:
Amteshwar Singh Sandhu
Lab Partner:
Imad Sadeq
Group Number:
B
Section:
FRI PM
PURPOSE
To use the steps of Gravimetric analysis technique and measure the % chloride in an
unknown, given quantity of salt.
THEORY
1. Equation of Formation of Silver Chloride:
𝐴𝑔!(!") + 𝐶𝑙!(!") → 𝐴𝑔𝐶𝑙(!) Eq. 1
This equation means that 𝐶𝑙! has an extra e-‐ in its valence shell to reach
stability. Whereas, 𝐴𝑔! has deficiency of one e-‐ to complete its valence shell
electrons and reach stability. Therefore, 𝐶𝑙! and 𝐴𝑔! share one e-‐ to fill their
valence shell electrons and attain stability. Thus, the result is the formation
of a stable compound 𝐴𝑔𝐶𝑙.
2. 𝐴𝑔𝐶𝑙 is highly insoluble in water. Thus, it forms precipitate as colloidal
particles. Ksp is the solubility product of the constituent ions. For example:
𝐴𝑔𝐶𝑙(!) → 𝐴𝑔!(!") + 𝐶𝑙!(!") Eq. 2
Ksp = 𝐴𝑔!(!") . 𝐶𝑙!(!") Eq. 3
Low solubility in aqueous solutions occurs when the Ksp has a lower value.
This, also means when the precipitate formed is maximum the solubility was
low in the aqueous solution.
3. Precipitation is carried out slowly in the presence of nitric acid to prevent
interference from other anions of weak acids. Thus, the precipitate formed is
free of co-‐precipitates.
4. By heating and stirring the solution gently during precipitation prevents the
formation of precipitate as colloidal particles. The precipitate formed is
proper crystal. Unlike the colloidal particles this, precipitate can be collected
using a filter setup.
5. There are two cases as follows:
o In Air:
The photodecomposition of silver chloride (precipitate) leads to the
formation of finely divided silver and chlorine gas:
𝐴𝑔𝐶𝑙(!) → 𝐴𝑔(!) + !!𝐶𝑙!(!) Eq. 4
The analytical results are low in this case. As the chlorine gas produced as
shown in Eq. 4 escapes the solution.
o When excess silver chloride is present in solution:
When excess silver ion present in the solution it reacts with chlorine gas and
forms silver chloride. Thus analytical results will have a higher value.
Eq. 5 shows the precipitation of silver chloride when there are silver ions
present in the solution.
3𝐶𝑙! ! + 3𝐻!𝑂 ! + 5𝐴𝑔! !" → 5𝐴𝑔𝐶𝑙 ! + 𝐶𝑙𝑂!!!" + 6𝐻!
!" Eq. 5
6. Because AgCl has very small amount of solubility in water. The maximum loss
can be accounted for using Ksp formula as:
The Ksp for AgCl is [1] 1.6×10!!"!"#!
The loss in moles when washed with 100 ml or 0.1 L of water will be:
1.6×10!!" !"#! × 0.1𝐿 = 1.6×10!!! 𝑚𝑜𝑙
Thus, the maximum precipitate loss in mass:
𝑚𝑎𝑠𝑠 = 𝑚𝑜𝑙𝑒𝑠 × 𝑀.𝑊. 𝑜𝑓 𝐴𝑔𝐶𝑙
𝑚𝑎𝑠𝑠 = 1.6 ×10!!! 𝑚𝑜𝑙 ×143.32 𝑔𝑚𝑜𝑙 = 2.3 ×10!! 𝑔
7. Ions such as CO3 2-‐ and CH3COO – form co-‐precipitates with silver and increase
the mass of the precipitate significantly.
PROCEDURE
Each partner performed the following procedure of the experiment individually. A
sample of salt #291 weighed as 0.1596 g and poured into a 250 ml beaker. The color
of the salt was white and the shape of the salt crystals was irregular. The sample salt
# 291 and crucibles were weighed as 0.1596 g and 31.3192 g with analytical balance.
The crucibles were weighted using a tissue to hold them and an error of ±0.0001 g
was accounted. 100 ml of distilled water and 1 ml of dilute 6M HNO3 were added to
the salt in a 250 ml beaker. Then the solution was heated and stirred slowly. Just
before the heating was started calculated amount of AgNO3 (30 mL) was added to
the salt (including 5ml extra). When the solution was almost transparent, the test to
check if the precipitation of silver chloride was complete was performed by adding
small amount of AgNO3. If more precipitate formed then, more AgNO3 was added
until precipitation was completed. When the precipitation was complete, solution
was stored in the drawer carefully without much moment of the solution.
Later, the vacuum filtration was setup and filter crucibles were used to Filter the
solution and the precipitate was collected. 5 ml of 0.01M HNO3 was used to remove
the remaining precipitate from the beaker and filtered through the crucible. Also,
water was used to remove the precipitate clung to the walls of the beaker. **5 ml of
washings were collected and add a small amount of HCL was added with help of TA.
If little or no turbidity was observed, the washing was complete. If cloudiness
appeared small amount of 0.01M HNO3 was filtered through the precipitate. The test
was performed again and if no turbidity appeared the washing was complete. When
the washing was complete then 5 ml Acetone was filtered through the precipitate
and collected separate, as this waste had to be treated before disposal.
The crucible was not placed in oven due to prior breakage. The mass of the
precipitate with crucible is *31.7216. The lab coordinator to complete the
calculations and formal report provides this mass.
** Please note – Due to the breakage of Crucibles prior to placing in oven, further
experiment was not performed and the procedure is written based on observing
partners experiment.
OBSERVATIONS
1. The sample number of the salt was #291. The salt was of white color and it
consisted of fine crystals.
2. A small amount of AgNO3 was added to the precipitate to complete the
precipitation. The test of washing the precipitate was performed using no
more than 5 ml 0.01M HNO3 but the test of HCL to check the washing was
not completed to the breakage of crucibles. Precipitate was lost due to
breakage.
3.
• The color of the precipitate was light violet. The color appeared
because certain exposure to light is inevitable. When the solution was
exposed to light silver chloride splits up into its constituent molecules
as shown in Eq. 4 above. The finely divided silver precipitate acquired
a violet color. Also, the silver precipitate was opaque in nature. Thus,
it prevented further decomposition unless the solution was steady.
• The shape of the precipitate was slightly powdery.
Partner’s data:
Physical description Completeness of precipitation
Washings with HNO3
Physical description of precipitate
White powder and fine shaped crystals
The precipitation was completed using 24 mL of AgNO3
The washing was successfully completed.
The precipitate was light purple and powdery after drying.
DATA
Values Partner’s values
Sample mass 0.1596 ± 0.0001 𝑔 0.1227± 0.0001 𝑔
Required AgNO3 30 mL 29 mL
Mass of crucible 31.3192 ± 0.0001 𝑔 32.6559± 0.0001 𝑔
Oven temperature N.A.* 111.0 ℃ ± 0.2 ℃
Time precipitate in oven N.A.* 30 minutes
Precipitate cooling time N.A.* 5 minutes
Mass of crucible with
precipitate
31.7216± 0.0001 𝑔 33.0079± 0.0001 𝑔
Mass precipitate 0.4024± 0.0002 𝑔 0.352± 0.0002 𝑔
*PLEASE NOTE: because the crucible broke prior to placing in oven. Lab coordinator provided the crucible with precipitate mass to complete the calculations and formal report.
DISCUSSION
The average value of the % Cl-‐ in solution is 66.7%, where as the true value is 56.19%. There are several reasons why average calculated value is higher than true value. Firstly, the most significant cause of this error could be addition of large excess of AgNO3 during precipitation. The precipitate must have entrapped anions of weak acids from the solution. Thus, the mass of the precipitate increased significantly due to impurities. Secondly, the result could be high because of improper washing of the precipitate during filtration. The anions of the weak acids might have been present in the precipitate even after washing. Also, during the HCl test *if little turbidity was observed that means a small amount of Ag is also accounted for in the precipitate. Other reasons for higher results could be uncertainty in weighing salts and measuring volume of chemicals. *PLEASE NOTE: because the crucible broke prior to HCl test. Lab coordinator provided the crucible with precipitate mass to complete the calculations and formal report.
CONCLUSION
1. The salt Sample #291 was used for the Gravimetric Analysis.
2. The average % Cl-‐ was 66.7% and the real value is 56.19%
3. The uncertainty for weighing sample salt was 6.2 ×10!! , uncertainty for weighing the precipitate is 4.9 ×10!! and uncertainty for oven temperature is *N.A. Partner’s Values for uncertainties: Uncertainty for weighing sample salt = 8.1 × 10!! , uncertainty for weighing precipitate was 5.6 ×10!! and the uncertainty for oven temperature was 1.8 ×10!!
4. The precision was 129 ppt
5. The relative error was 11% Partner’s relative error was – 26.4%
BIBLIOGRAPHY
1. Archer D.W., Burk R.C., White C.A., Wolff P.A. and Levac S. ‘Gravimetric Analysis of a Chloride Salt’, CHEM 1101: Chemistry for engineers Laboratory Lab manual, Carleton University, Ottawa, Fall 2013, 54-‐59. *PLEASE NOTE: because the crucible broke prior to placing in oven. Lab coordinator provided the crucible with precipitate mass to complete the calculations and formal report.
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