Comparison of Two Chloride Determinations

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Matt Marthaler
Experiment Four
Comparison of Two Chloride Determinations
Purpose:
In this lab, I will compare the use of two different β€œwet” chemical techniques for the analysis of
an unknown chloride sample. The first analysis is a volumetric chloride determination and the second is
a gravimetric determination. I will then compare these methods statistically for the percent chloride
determination.
Procedure:
Volumetric Chloride Determination
1) Place about 4 g of AgNO3 in 100 mL of distilled water and let it dissolve. Place this now into a
200 mL volumetric flask and dilute to the mark followed by a thorough mixing.
2) Into 3 250 mL Erlenmeyer flasks, weigh about 0.1 g of the unknown chloride. Dissolve each
sample in 50 mL of water. Add 0.03 g dextrin and 5 drops of dichloroflurescein indicator.
Titrate the solution with silver nitrate to a pink endpoint.
3) Calculate the percent chloride in the sample.
Gravimetric Chloride determination
1) Prepare 3 Gooch crucibles by cleaning with an acid (0.1 M HNO3) and then base (0.1 M NH3).
Follow this with a wash by distilled water and then place in a 120 degree oven for an hour.
Fit each crucible with a glass fiber filter.
2) Dissolve each sample into a 400 mL beaker with 150 mL of distilled water, then add one mL
of 6 M HNO3.
3) Add silver nitrate to precipitate the solution. Heat this without boiling until the supernatant
is clear.
4) Filter samples through Gooch crucibles with mild suction. Use warm (0.1 M HNO3) to wash
the precipitate five times. Rinse the precipitate with distilled water after this.
5) Dry the crucibles and precipitate in a 120 degree oven for at least two hours before a final
weighing.
Reactions:
Cl- + AgNO3
AgCl(s)
AgCl(s) + NO3Ag+ + Cl-
Data:
Gravimetric Analysis
Mass of Gooch and Lid Before
Wash (g)
33.0188
34.1303
35.5765
34.2664
Sample
751
2
3
Average
Sample
Mass of
Unknown
sample (g)
Mass of Filter
Paper (g)
1
2
3
Average
0.2009
0.1999
0.2006
0.2005
0.0288
0.0297
0.0301
0.0295
Mass of Gooch,
Lid, and
Product After
Heating (g)
33.4928
34.5552
36.0559
34.7013
Mass of Gooch and Lid After
Wash (g)
33.0172
34.1281
35.5763
34.2405
Mass of Cl(g)
% Cl in
Unknown
0.1105
0.0983
0.1111
0.1067
55.01
49.17
55.43
53.20
Volumetric Determination
Sample
1
2
3
Average
Unknown Mass
(g)
0.1012
0.1004
0.1004
0.1007
Dextrin Mass
(g)
0.0304
0.0300
0.0310
0.0305
mL of AgNO3
used to Titrate
14.40
13.99
13.11
13.83
Mass of Cl- (g)
0.06021
0.05850
0.05482
0.05784
% Cl- in
Unknown
59.50%
58.27%
54.60%
57.46%
AgNO3 mass and mols: 4.0091 g and 0.118 mols
Comparison of the Two Methods
Sample
1
2
3
Average
Standard Deviation: 4.97
Tcalculated: 1.48
Vol. Determination %
Cl
59.50
58.27
54.60
57.46
Grav. Determination%
Cl
55.01
49.17
55.43
53.20
Difference
4.49
9.10
-0.83
4.26
Calculations:
Volumetric Analysis
Moles of AgNO3 used:
4.00091 g AgNO3 x 1000 mL x 1 mol AgNO3
=
200 mL x 1 L x 169.87 g AgNO3
Mass of Cl- (Final):
14.40 mL AgNO3 x 4.0091 g AgNO3 x 1 mol AgNO3 x 1 mol Cl x 35.435 g Cl
=
200 ml AgNO3 x 169.8682 g AgNO3 x 1 mol AgNO3 x 1 mol Cl
% Cl- in Unknown:
0.0602 𝑔 𝐢𝑙
0.1012 𝑔 π‘ˆπ‘›π‘˜π‘›π‘œπ‘€π‘›
0.118 mol AgNO3
0.0602 g Cl-
x 100% = 59.50%
Gravimetric Analysis
Volume of AgNO3 needed to precipitate the unknown chloride:
g unk sample x mol AgNO3 x L AgNO3 x avg %Cl from volumetric analysis
mol Cl2 x mol AgNO3 x g Cl
Ex:
0.2009 g unk sample x 1mol AgNO3 x 1 L AgNO3 x .5746 %Cl from volumetric analysis
1 mol Cl2 x 0.118 mol AgNO3 x g Cl
= 27.61 mL AgNO3
Mass of Product (AgCl): (mass of crubile, lid, and product) – (mass of crucible and lid + filter paper)
Ex:
(33.4928 g) – (33.0172 + 0.0288) = 0.4468g AgCl
Mass of Cl-final:
0.4468 g AgCl x 1 mol AgCl+1 mol Cl x 35.45 g Cl
=
143.32 g AgCl x 1 mol AgCl x 1 mol Cl
% Cl- in Unknown:
0.1105 g Clβˆ’
x
0.2009 g AgCl
0.1105 g Cl-
100% = 55.01%
Comparison of the Two Methods
Standard Deviation of differences:
βˆ‘i (diβˆ’mean ds)2
nβˆ’1
s=√
βˆ‘(4.49βˆ’4.26)2 +(9.10βˆ’4.26)2 +(βˆ’.83βˆ’4.26)2
3βˆ’1
Example:
s= √
Ttest:
tcalc= |𝑑𝑠 | ×βˆšπ‘›
Ex:
tcalc = 4.26
π‘₯ √3 = 1.48
4.97
Μ…
= 4.97
Conclusion:
The reason this lab was performed was to determine whether the results of %Cl using
volumetric and gravimetric determination were comparable. After calculation, we have found that the
%Cl was 57.46% from the volumetric determination and 53.20% from the gravimetric determination. In
comparing these two results using a t test, I have calculated t to be 1.48. When this is compared to the t
table value at the 95% confidence level (3.182), the two methods were found to be not significantly
different. Some procedural changes that would improve the error are to have better cleaning
procedures of the Gooch crucibles to get rid of all the impurities and to make sure the oven door was
closed the whole time to dry the product fully.
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