Experiment Four - Comparison of Two Chloride Determinations

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Orlando 1
Quantitative Analytical Chemistry
Joseph Orlando
Lab #4: Comparison of Two Chloride Determinations
Purpose:
In this experiment we analyzed an unknown chloride sample via two different β€œwet” chemical
techniques; a volumetric chloride determination and a gravimetric determination. After the
experimental data was obtained, the two methods were statistically compared.
Cl- + AgNO3 -----> AgCl (s) + NO3AgCl (s) -----> Ag+ + ClProcedure:
Volumetric Chloride Determination
1. Weigh ~4g AgNO3 and dissolve in 100mL distilled H2O. Transfer solution to a 200mL
volumetric flask and dilute and mix.
2. Into a 125mL Erlenmeyer flask weigh out 0.1g of unknown chloride. Dissolve three samples
in 50mL H2O. Add 0.03g dextrin and 5 drops dichlorofluorescein indicator. Titrate with silver
nitrate solution to a pink endpoint.
3. Calculate percent chloride in the sample.
Gravimetric Chloride Determination
1. Clean three gooch crucibles with 0.1 M HNO3 then with 0.1 M NH3. Follow with a distilled
H2O wash then dry for an hour in a 120° C oven. Fit each crucible with a glass fiber filter.
2. Dissolve three samples (0.2g) in a 400mL beaker with 150mL H2O, add 1mL of 6M HNO3.
3. Add enough silver nitrate to precipitate unknown chloride. Heat on a hot plate without
boiling and frequent stirring.
4. Filter samples through gooch crucibles with suction. Use warm 0.1M HNO3 to was
precipitate. 5 washes suggested. Rinse with distilled H2O.
5. Dry crucibles and precipitate in a 120°C oven for two hours, weigh.
Data:
Volumetric Determination
Mass (g) of dextrin
added
Volume (L) of
AgNO3 titrated
Percent (%)
Chloride
Trial #1
Mass (g) of
Unknown Chloride
added
0.1157
0.0308
0.01472
54.70%
Trial #2
0.1087
0.0305
0.01449
57.37%
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Trial #3
0.1045
0.0296
0.01369
56.38%
Average
0.1096
0.3030
0.01430
56.15%
Gravimetric Determination
Mass (g) of
Product
0.4808
Mass (g) Cl
Trial #1
Mass (g)
Unknown Sample
0.2299
0.1189
Percent (%)
Chloride
51.78%
Trial #2
0.2350
0.5159
0.1276
54.30%
Trial #3
0.2051
0.4358
0.1077
52.51%
Average
0.2233
0.4775
0.1181
53.41%
Student’s T-test
Gravimetric
Determination Cl- %
51.78%
Difference
Trial #1
Volumetric
Determination Cl- %
54.70%
Trial #2
57.37%
54.30%
3.07
Trial #3
56.38%
52.51%
3.87
2.98
Average
3.31
Standard Deviation
0.499
tcalculated
3.04
π‘‘π‘‘π‘Žπ‘π‘™π‘’
4.303 (at 95% Confidence Interval)
Equations:
Mass of Cl-final:
amount of
𝐴𝑔𝑁𝑂3 titrated ×
4.0126 AgNO3
1 π‘šπ‘œπ‘™ AgNO3
1 π‘šπ‘œπ‘™ 𝐢𝑙
35.45 𝑔 𝐢𝑙
×
×
×
=mass of Cl
200 mL AgNO3 169.868 𝑔 AgNO3 1 π‘šπ‘œπ‘™ AgNO3 1 π‘šπ‘œπ‘™ 𝐢𝑙
Percent Cl- in Unknown:
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π‘šπ‘Žπ‘ π‘  π‘œπ‘“πΆπ‘™
× 100% = π‘π‘’π‘Ÿπ‘π‘’π‘›π‘‘ π‘β„Žπ‘™π‘œπ‘Ÿπ‘–π‘‘π‘’ 𝑖𝑛 π‘ π‘Žπ‘šπ‘π‘™π‘’
mass of unknown Cl used
Gravimetric Determination
Mass of Cl-final:
mass of AgCl×
1 π‘šπ‘œπ‘™ 𝐴𝑔𝐢𝑙
1 π‘šπ‘œπ‘™ 𝐢𝑙
35.45 𝑔 𝐢𝑙
×
×
= π‘šπ‘Žπ‘ π‘  π‘œπ‘“πΆπ‘™
143.32 𝑔 𝐴𝑔𝐢𝑙 1 π‘šπ‘œπ‘™ 𝐴𝑔𝐢𝑙
1 π‘šπ‘œπ‘™ 𝐢𝑙
% Cl- in Unknown:
mass of Cl
×100% = percent chloride in sample
mass of unknown sample used
Student’s t Test:
π‘‘π‘π‘Žπ‘™π‘ =
|𝑑̅|
×βˆšπ‘›
𝑠
Conclusion
The average chloride % that was calculated from the volumetric method was 56.15% and
53.41% from the gravimetric method. At the 95% Confidence interval with 2 degrees of freedom
the π‘‘π‘‘π‘Žπ‘π‘™π‘’ value is 4.303. Using the student’s T-test it was calculated that T value for the two
different methods was 3.04. Because the π‘‘π‘π‘Žπ‘™π‘ value is less than the π‘‘π‘‘π‘Žπ‘π‘™π‘’ value we can say that
there is less than a 95% chance that the data from the two methods, volumetric and gravimetric,
are different. This experiment proved to be an overall success.
Post Lab Questions:
1. Discuss appropriate procedural changes which could improve the error in both experiments.
The main procedural change would involve the drying process in the furnace for two hours. I
believe that the product was not allowed to fully dry and therefore this altered the data that
was obtained.
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