Quant Lab 4

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Experiment 4: Comparison of Two Chloride Determinations
Introduction:
To compare 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.
Procedure:
Volumetric Chloride Determination
1. 4.5g of AgNO3 was dissolved and diluted in a 250 mL volumetric flask.
2. Standardize the AgNO3 using oven dried sodium chloride. Weigh enough sodium
chloride for a 25 mL titration. (0.155g NaCl)
3. Add 0.03 g of dextrin and 5 drops of dichlorofluoresein as an indicator. Titrate solution to
a pink endpoint. Obtain three good trials.
Gravimetric Chloride Determination
1. Clean three Gooch crucibles with an acid solution (0.1M HNO3) and then (0.1M
NH3) basic.
2. Follow washes with distilled water and dry one hour in 120°C oven. Let crucibles
cool and fit each crucible with a glass fiber filter.
3. Accurately weigh 0.2 g unknown chloride and dissolve each sample in a 400 mL
beaker with 150 mL distilled water. Add 1 mL of 6M HNO3.
4. Add silver nitrate solution to completely precipitate the unknown chloride. Heat the
solution on a hot plate without boiling and frequently stir until the supernatant is
clear.
5. Vacuum filter the samples through weighed Gooch crucibles. Use warm 0.1M HNO3
to wash the precipitate five times, and then finally wash the precipitate with distilled
water.
6. Dry the crucibles and precipitate in a 120°C oven for at least two hours before final
weighing.
Reactions:
AgNO3 + Cl- οƒ AgCl (s) + NO3AgCl (s) οƒ  Ag+ + ClData:
Mass AgNO3= 4.5006g
Amount dextrin
0.0328g
0.0336g
0.0390g
Unknown
chloride
0.1041g
0.1002g
0.1078g
Amount AgNO3
solution
16.00mL
14.60mL
15.00mL
Part 2 Data:
Mass unknown
chloride
Amount AgNO3 added
Mass crucible and
filter paper
Mass crucible, filter
paper, and product
Mass product
Trial 1
0.2016g
Trial 2
0.2028g
Trial 3
0.2014g
7.3mL
21.1403g
9.5mL
22.3275g
8.4mL
22.6890g
21.3759g
22.6776g
22.9440g
0.2356g
0.3501g
0.2550g
Calculated Data(Volumetric)
Trial
1
2
3
M AgNO3
0.106
0.106
0.106
Mol AgNO3 & Cl0.0017
0.0015
0.00159
Mass Cl- in Precipitate (g)
0.0601
0.0532
0.0564
Mass % Cl- in Uknown
57.7%
53.1%
52.3%
Calculated Data (Gravimetric & Corrected)
Trial
Mass Clin precip.
(g)
Mass % Clin
unknown
1
2
3
0.0583
0.0866
0.0631
28.9%
42.7%
31.3%
Theroetical
AgNO3
needed
(mL)
30.4
30.4
30.4
Fraction
AgCl
obtained
Total
mass AgCl
(g)
Total
mass
Cl- (g)
0.2401
0.3125
0.2763
0.9813
1.1203
0.9229
0.2427
0.2771
0.2283
Mass % Clin
Unknown
(corrected)
120.4%
136.6%
113.4%
Calculations:
Volumetric Determination
Molarity of AgNO3
1 π‘šπ‘œπ‘™ 𝐴𝑔𝑁𝑂3
𝑀 = π‘šπ‘Žπ‘ π‘ π΄π‘”π‘π‘‚3 ×
÷𝑉
169.873𝑔𝐴𝑔𝑁𝑂3
Moles of AgNO3 & Cl𝑀 𝐴𝑔𝑁𝑂3 × βˆ†π‘‰ 𝐴𝑔𝑁𝑂3 = π‘šπ‘œπ‘™ 𝐴𝑔𝑁𝑂3 & 𝐢𝑙 βˆ’
Mass of Cl- in Precipitate
35.453𝑔𝐢𝑙 βˆ’
π‘šπ‘œπ‘™ 𝐢𝑙 βˆ’ ×
= π‘š 𝐢𝑙 βˆ’
1 π‘šπ‘œπ‘™ 𝐢𝑙 βˆ’
Mass Percent of Cl- in Unknown
π‘š 𝐢𝑙 βˆ’
× 100% = π‘šπ‘Žπ‘ π‘  % 𝐢𝑙 βˆ’
π‘š 𝑖𝑛𝑖𝑑. π‘ π‘Žπ‘šπ‘π‘™π‘’
Molarity of AgNO3
1 π‘šπ‘œπ‘™
÷ 0.250𝐿
169.873𝑔
Moles of AgNO3 & Cl- Trial 1
0.106𝑀 × 0.01600𝐿 = 0.0017 π‘šπ‘œπ‘™
Mass of Cl- in Precipitate Trial 1
35.453𝑔𝐢𝑙 βˆ’
0.0017π‘šπ‘œπ‘™πΆπ‘™ βˆ’ ×
= 0.0601𝑔𝐢𝑙 βˆ’
1 π‘šπ‘œπ‘™ 𝐢𝑙 βˆ’
Mass Percent of Cl- in Unknown Trial 1
0.0601𝑔 𝐢𝑙 βˆ’
× 100% = 57.7% 𝐢𝑙 βˆ’
0.1041𝑔 π‘ˆπΎ
0.106𝑀 = 4.5006𝑔 ×
Gravimetric Determination
Mass of AgCl (precipitate)
π‘šπ‘“π‘–π‘›π‘Žπ‘™ βˆ’ π‘šπ‘π‘Ÿπ‘’π‘. βˆ’ π‘šπ‘“ π‘π‘Žπ‘π‘’π‘Ÿ = π‘š 𝐴𝑔𝐢𝑙
Mass of Cl- in precipitate
1 π‘šπ‘œπ‘™
1 π‘šπ‘œπ‘™ 𝐢𝑙 βˆ’
π‘š 𝐴𝑔𝐢𝑙 ×
×
143.321𝑔 1 π‘šπ‘œπ‘™ 𝐴𝑔𝐢𝑙
35.453𝑔
×
= π‘š 𝐢𝑙 βˆ’
1 π‘šπ‘œπ‘™
Mass Percent of Cl- in Unknown
π‘š 𝐢𝑙 βˆ’
× 100% = π‘šπ‘Žπ‘ π‘  % 𝐢𝑙 βˆ’
π‘š 𝑖𝑛𝑖𝑑. π‘ π‘Žπ‘šπ‘π‘™π‘’
Correction Gravimetric Determination Data
Theoretical amount of AgNO3 needed
Μ…Μ…Μ…Μ… AgNO3 × 2 β‰… 𝑉 𝐴𝑔𝑁𝑂3 𝑛𝑒𝑒𝑑𝑒𝑑
βˆ†V
Fraction of AgCl obtained
𝑉 𝐴𝑔𝑁𝑂3 𝑒𝑠𝑒𝑑
𝑉 𝐴𝑔𝑁𝑂3 𝑛𝑒𝑒𝑑𝑒𝑑
= π‘“π‘Ÿπ‘Žπ‘π‘‘π‘–π‘œπ‘› π‘œπ‘“ 𝐴𝑔𝐢𝑙 π‘œπ‘π‘‘π‘Žπ‘–π‘›π‘’π‘‘
Total mass of AgCl
π‘š 𝐴𝑔𝐢𝑙 π‘œπ‘π‘‘π‘Žπ‘–π‘›π‘’π‘‘
= π‘‘π‘œπ‘‘π‘Žπ‘™ π‘š 𝐴𝑔𝐢𝑙
π‘“π‘Ÿπ‘Žπ‘π‘‘π‘–π‘œπ‘› π‘œπ‘“ 𝐴𝑔𝐢𝑙 π‘œπ‘π‘‘π‘Žπ‘–π‘›π‘’π‘‘
Total mass of Cl1 π‘šπ‘œπ‘™
1 π‘šπ‘œπ‘™ 𝐢𝑙 βˆ’
π‘šπ‘‘π‘œπ‘‘ 𝐴𝑔𝐢𝑙 ×
×
143.321𝑔 1 π‘šπ‘œπ‘™ 𝐴𝑔𝐢𝑙
35.453𝑔
×
= π‘š 𝐢𝑙 βˆ’
1 π‘šπ‘œπ‘™
Mass Percent of Cl- in Unknown
π‘š 𝐢𝑙 βˆ’
× 100% = π‘šπ‘Žπ‘ π‘  % 𝐢𝑙 βˆ’
π‘š 𝑖𝑛𝑖𝑑. π‘ π‘Žπ‘šπ‘π‘™π‘’
t calc =
Mass of AgCl (precipitate) Trial 1
21.3759𝑔 βˆ’ 21.1403𝑔 = 0.2356𝑔 𝐴𝑔𝐢𝑙
Mass of Cl- in precipitate Trial 1
1 π‘šπ‘œπ‘™
1 π‘šπ‘œπ‘™ 𝐢𝑙 βˆ’
0.2356𝑔 ×
×
143.321𝑔 1 π‘šπ‘œπ‘™ 𝐴𝑔𝐢𝑙
35.453𝑔
×
= 0.0583𝑔 𝐢𝑙 βˆ’
1 π‘šπ‘œπ‘™
Mass Percent of Cl- in Unknown Trial 1
0.0583𝑔 𝐢𝑙 βˆ’
× 100% = 28.91% 𝐢𝑙 βˆ’
0.2016𝑔 π‘ˆπΎ
Theoretical amount of AgNO3 needed Trial
1
15.20mL AgNO3 × 2 β‰… 30.40π‘šπΏ 𝐴𝑔𝑁𝑂3
Fraction of AgCl obtained Trial 1
0.0073𝐿 𝐴𝑔𝑁𝑂3 𝑒𝑠𝑒𝑑
0.03040𝐿 𝐴𝑔𝑁𝑂3 𝑛𝑒𝑒𝑑𝑒𝑑
= .2401 𝐴𝑔𝐢𝑙 π‘œπ‘π‘‘π‘Žπ‘–π‘›π‘’π‘‘
Total mass of AgCl Trial 1
0.2356𝑔 𝐴𝑔𝐢𝑙 π‘œπ‘π‘‘π‘Žπ‘–π‘›π‘’π‘‘
= .9813𝑔 𝐴𝑔𝐢𝑙
0.2401 𝐴𝑔𝐢𝑙 π‘œπ‘π‘‘π‘Žπ‘–π‘›π‘’π‘‘
Total mass of Cl- Trial 1
1 π‘šπ‘œπ‘™
1 π‘šπ‘œπ‘™ 𝐢𝑙 βˆ’
0.9813𝑔 𝐴𝑔𝐢𝑙 ×
×
143.321𝑔 1 π‘šπ‘œπ‘™ 𝐴𝑔𝐢𝑙
35.453𝑔
×
= 0.2427𝑔 𝐢𝑙 βˆ’
1 π‘šπ‘œπ‘™
Mass Percent of Cl- in Unknown Trial 1
0.2427𝑔 𝐢𝑙 βˆ’
× 100% = 120.4% 𝐢𝑙 βˆ’
0.2016𝑔
|𝑑̅ |
√n
𝑠
Volumetric %
Gravimetric %
Difference
57.7
28.9
28.8
53.1
42.7
10.4
52.3
31.3
21.0
20.07
× βˆš6 = 5.32
9.235
ttable for at 95% confidence interval is = 2.776
5.32 > 2.776
Discussion:
1. We compared data with the student’s t test. Both methods are fairly accurate and give
similar results. However, the gravimetric data seems to be more precise than the
volumetric. The mass percent of our Cl- is well over 100% which means we could have
made an error during our experiment, and more trials should be done to test the unknown.
2. We performed a direct titration because the titrant was added directly to the known
analyte.
3. The gravimetric determination was indirect because the analyte concentration was
unknown.
Conclusion:
1. To improve this experiment we could use a better indicator that gives a more distinctive
color change to make sure we don’t over titrate. Another improvement could be to make
sure that the crucibles were measured at the exact same temperature at the final weighing.
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