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48725235-EXPERIMENT-4-28-1

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Name: Prem Patrick Parcon
Date performed: Jan. 18, 2011
Co-worker: Kate Alyssa Caton
Date submitted: Jan. 25, 2011
Experiment No. 4
PRECIPITATION TITRATIONS
DETERMINATION OF THE CHLORIDE IN THE SAMPLE
I. THEORETICAL FRAMEWORK
Precipitation titrimetry is one of the oldest analytical techniques, dating back to the mid 1800s.
The most widely used and most important precipitating reagent is silver nitrate. Precipitation titration is
sometimes based on reactions that yield ionic compounds of limited solubility. Titrimetric methods
based upon silver nitrate are sometimes termed as argentometric methods. This experiment also
involves the use of the Mohr method. For concentrations lower than 0.1 M, an indicator blank may also
be run to compensate for the overconsumption of reagent and for the acuity of the analyst in detecting
the appearance of color.
II. DATA AND RESULTS
Standardization of Silver Nitrate Solution
Primary Standard used:
NaCl
Formula mass of 1O standard:
58.44 g/mol
% Purity of 1O standard:
99.5%
Trials
Mass of NaCl (g)
1
2
3
0.1990
0.1850
0.1769
Final Reading AgNO3
(ml)
37.50
31.80
34.66
Initial Reading AgNO3
(ml)
6.21
1.19
5.45
Volume AgNO3 used
(ml)
31.29
30.79
29.21
30.82
31.33
34.19
Corrected Volume of AgNO3, mL
Molarity of AgNO3
0.1105
Average Molarity of AgNO3
Titer NaCl
(mg/mL)
0.1010
0.0885
0.1000 ± 0.110
6.457
Average Titer NaCl (mg/mL)
5.905
5.174
5.841 ± 0.6436
Indicator Blank
Trials
1
2
3
Final volume AgNO3
(ml)
32.80
33.00
33.40
Initial volume AgNO3
(ml)
32.0
32.80
33.00
Net Volume AgNO3
(ml)
0.80
0.20
0.40
Average volume of AgNO3, (ml)
0.47 ± 0.305
Analysis of Unknown
Trials
Mass of the sample
(g)
1
2
3
0.1597
0.1664
0.1567
Final volume AgNO3
(ml)
30.19
44.51
17.85
Initial volume AgNO3
(ml)
17.85
30.19
5.50
Net Volume AgNO3
(ml)
12.34
14.32
12.35
Corrected volume of AgNO3, (ml)
11.87
13.85
11.88
mg Cl- of unknown
42.08
49.10
42.11
% Cl- of unknown
26.35%
29.51%
26.87%
Average % Cl- of unknown
III. CALCULATIONS
27.58 ± 1.694
π‘Šπ‘’π‘–π‘”β„Žπ‘‘ π‘œπ‘“ 𝐢𝑙 π‘₯ %π‘ƒπ‘’π‘Ÿπ‘–π‘‘π‘¦
= 𝑁 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 × π‘‰ π‘œπ‘“ 𝐴𝑔𝑁𝑂3
πΈπ‘žπ‘’π‘–π‘£π‘Žπ‘™π‘’π‘›π‘π‘’ π‘Šπ‘’π‘–π‘”β„Žπ‘‘
0.1990 × 0.995
58.4428
Trial 1
= 𝑁 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 π‘₯ 0.03082
𝑁 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 = 0.1099
0.1850 × 0.995
= 𝑁 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 π‘₯ 0.03133
58.4428
Trial 2
𝑁 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 = 0.10053
0.1769 × 0.995
= 𝑁 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 π‘₯ 0.03419
58.4428
Trial 3
𝑁 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 = 0.0881
πΆπ‘œπ‘Ÿπ‘Ÿπ‘’π‘π‘‘π‘’π‘‘ π‘‰π‘œπ‘™π‘’π‘šπ‘’ = π‘‰π‘œπ‘™π‘’π‘šπ‘’ 𝐴𝑔𝑁𝑂3 𝑒𝑠𝑒𝑑 − 𝐴𝑣𝑒. π‘‰π‘œπ‘™. π‘œπ‘“ 𝐴𝑔𝑁𝑂3 (πΌπ‘›π‘‘π‘–π‘π‘Žπ‘‘π‘œπ‘Ÿ π΅π‘™π‘Žπ‘›π‘˜)
Trial 1
1.29 mL– 0.47 mL = 30.82 mL
Trial 2
31.80 mL – 0.47 mL = 31.33 mL
Trial 3
34.66 mL – 0.47 mL = 34.19 mL
π‘Šπ‘’π‘–π‘”β„Žπ‘‘ π‘π‘ŽπΆπ‘™
= 𝑀 π‘œπ‘“ 𝐴𝑔𝑁𝑂3 × πΆπ‘œπ‘Ÿπ‘Ÿπ‘’π‘π‘‘π‘’π‘‘ π‘‰π‘œπ‘™π‘’π‘šπ‘’ π‘œπ‘“ 𝐴𝑔𝑁𝑂3 (𝑖𝑛 𝐿)
π‘€π‘œπ‘™π‘’π‘π‘’π‘™π‘Žπ‘Ÿ π‘Šπ‘’π‘–π‘”β„Žπ‘‘ π‘π‘ŽπΆπ‘™
Trial 1
0.1990 𝑔
= 𝑀 × 0.03082 𝐿
58.4428
𝑀 = 0. 1105 𝑀
Trial 2
0.1850 𝑔
= 𝑀 × 0.03133 𝐿
58.4428
𝑀 = 0.1010 𝑀
Trial 3
0.1769 𝑔
= 𝑀 × 0.03419 𝐿
58.4428
𝑀 = 0.0885 𝑀
π΄π‘£π‘’π‘Ÿπ‘Žπ‘”π‘’ π‘€π‘œπ‘™π‘Žπ‘Ÿπ‘–π‘‘π‘¦ π‘œπ‘“ π΄π‘”π‘π‘œ3 =
0.1105 + 0.1010 + 0.0885
= 0.1000 𝑀
3
π‘šπ‘”
π‘Šπ‘’π‘–π‘”β„Žπ‘‘ π‘π‘ŽπΆπ‘™ (𝑖𝑛 π‘šπ‘”)
π‘‡π‘–π‘‘π‘’π‘Ÿ π‘π‘ŽπΆπ‘™ ( ) =
π‘šπΏ
πΆπ‘œπ‘Ÿπ‘Ÿπ‘’π‘π‘‘π‘’π‘‘ π‘‰π‘œπ‘™π‘’π‘šπ‘’ 𝐴𝑔𝑁𝑂3
Trial 1
1000 π‘šπ‘”
1𝑔
π‘‡π‘–π‘‘π‘’π‘Ÿ π‘π‘ŽπΆπ‘™ =
30.82
π‘šπ‘”
= 6.457
π‘šπΏ
Trial 2
1000 π‘šπ‘”
1𝑔
π‘‡π‘–π‘‘π‘’π‘Ÿ π‘π‘ŽπΆπ‘™ =
31.33
π‘šπ‘”
= 5.905
π‘šπΏ
Trial 3
1000 π‘šπ‘”
1𝑔
π‘‡π‘–π‘‘π‘’π‘Ÿ π‘π‘ŽπΆπ‘™ =
34.19 π‘šπΏ
π‘šπ‘”
= 5.174
π‘šπΏ
0.1990 𝑔 ×
0.1850 𝑔 ×
0.1769 𝑔 ×
π΄π‘£π‘’π‘Ÿπ‘Žπ‘”π‘’ π‘‡π‘–π‘‘π‘’π‘Ÿ =
6.457 + 5.905 + 5.174
= 5.845 π‘šπ‘”/π‘šπΏ
3
πΆπ‘œπ‘Ÿπ‘Ÿπ‘’π‘π‘‘π‘’π‘‘ π‘‰π‘œπ‘™π‘’π‘šπ‘’ = 𝑁𝑒𝑑 π‘‰π‘œπ‘™π‘’π‘šπ‘’ 𝐴𝑔𝑁𝑂3 − 𝐴𝑣𝑒. π‘‰π‘œπ‘™. π‘œπ‘“ 𝐴𝑔𝑁𝑂3 (πΌπ‘›π‘‘π‘–π‘π‘Žπ‘‘π‘œπ‘Ÿ π΅π‘™π‘Žπ‘›π‘˜)
Trial 1
12.34 mL– 0.80 mL = 11.87 mL
Trial 2
14.32 mL – 0.20 mL = 13.85 mL
Trial 3
12.35 mL – 0.40 mL = 11.88 mL
π‘šπ‘–π‘™π‘™π‘–π‘šπ‘œπ‘™π‘’ 𝐢𝑙 = 𝑀 π‘œπ‘“ π΄π‘”π‘π‘œ3 × πΆπ‘œπ‘Ÿπ‘Ÿπ‘’π‘π‘‘π‘’π‘‘ π‘‰π‘œπ‘™π‘’π‘šπ‘’ π‘œπ‘“ π‘π‘ŽπΆπ‘™ (π‘šπΏ)
Trial 1
π‘šπ‘šπ‘œπ‘™π‘’ 𝐢𝑙 = 𝐴𝑣𝑒. π‘€π‘œπ‘™π‘Žπ‘Ÿπ‘–π‘‘π‘¦ 𝐴𝑔𝑁𝑂3 × π‘‰π‘œπ‘™π‘’π‘šπ‘’ π΄π‘”π‘π‘œ3
= 0.1000 𝑀 × 11.87 π‘šπΏ = 1.187 π‘šπ‘šπ‘œπ‘™π‘’π‘ 
Trial 2
π‘šπ‘šπ‘œπ‘™π‘’ 𝐢𝑙 = 𝐴𝑣𝑒. π‘€π‘œπ‘™π‘Žπ‘Ÿπ‘–π‘‘π‘¦ 𝐴𝑔𝑁𝑂3 × π‘‰π‘œπ‘™π‘’π‘šπ‘’ π΄π‘”π‘π‘œ3
= 0.1000 𝑀 × 13.85 π‘šπΏ = 1.385 π‘šπ‘šπ‘œπ‘™π‘’π‘ 
Trial 3
π‘šπ‘šπ‘œπ‘™π‘’ 𝐢𝑙 = 𝐴𝑣𝑒. π‘€π‘œπ‘™π‘Žπ‘Ÿπ‘–π‘‘π‘¦ 𝐴𝑔𝑁𝑂3 × π‘‰π‘œπ‘™π‘’π‘šπ‘’ π΄π‘”π‘π‘œ3
= 0.1000 𝑀 × 11.88 π‘šπΏ = 1.188 π‘šπ‘šπ‘œπ‘™π‘’π‘ 
π‘šπ‘” 𝐢𝑙 = π‘šπ‘šπ‘œπ‘™π‘’ 𝐢𝑙 ×
π‘€π‘œπ‘™π‘’π‘π‘’π‘™π‘Žπ‘Ÿ π‘Šπ‘’π‘–π‘”β„Žπ‘‘ 𝐢𝑙 (π‘šπ‘”)
1π‘šπ‘šπ‘œπ‘™
π‘šπ‘” 𝐢𝑙 = 1.187 π‘šπ‘šπ‘œπ‘™π‘’π‘  ×
Trial 1
35.45 π‘šπ‘” 𝐢𝑙
1 π‘šπ‘šπ‘œπ‘™
= 42.08 π‘šπ‘”
π‘šπ‘” 𝐢𝑙 = 1.385 π‘šπ‘šπ‘œπ‘™π‘’π‘  ×
Trial 2
35.45 (π‘šπ‘”)
1 π‘šπ‘šπ‘œπ‘™
= 49.10 π‘šπ‘”
π‘šπ‘” 𝐢𝑙 = 1.188 π‘šπ‘šπ‘œπ‘™π‘’π‘  ×
Trial 3
35.45 (π‘šπ‘”)
1 π‘šπ‘šπ‘œπ‘™
= 42.11 π‘šπ‘”
%𝐢𝑙 π‘œπ‘“ π‘’π‘›π‘˜π‘›π‘œπ‘€π‘› π‘ π‘Žπ‘šπ‘π‘™π‘’ =
π‘šπ‘” 𝐢𝑙
× 100
π‘šπ‘Žπ‘ π‘  π‘ π‘Žπ‘šπ‘π‘™π‘’ (π‘šπ‘”)
Trial 1
%𝐢𝑙 =
42.08 π‘šπ‘”
× 100 = 26.35%
159.7 π‘šπ‘”
Trial 2
%𝐢𝑙 =
49.10 π‘šπ‘”
× 100 = 24.51%
166.4 π‘šπ‘”
Trial 3
%𝐢𝑙 =
42.11 π‘šπ‘”
× 100 = 26.87%
156.7 π‘šπ‘”
π΄π‘£π‘’π‘Ÿπ‘Žπ‘”π‘’ % 𝐢𝑙 π‘œπ‘“ π‘’π‘›π‘˜π‘›π‘œπ‘€π‘› =
26.35 + 24.51 + 26.87
= 25.91%
3
IV. DISCUSSION AND INTERPRETATION OF RESULTS
The main goal of this precipitation titration experiment involved the determination of the
chloride in the sample with silver nitrate as the indicator. First, 250 ml of 0.1 M AgNO3 solution was
prepared from 1.0 M solution. To know the volume of silver nitrate needed the equation:
1.0 𝑀 × π‘‰ = 0.1 𝑀 × 250 π‘šπ‘™
𝑉 = 25.0 π‘šπ‘™ 𝐴𝑔𝑁𝑂3
Next, silver nitrate was standardized using Mohr method. Three samples of NaCl weighing at
least 0.15 g to 0.20 g was measured and then dissolved in 50 ml water. After which, 2 ml of potassium
chromate was added in each Erlenmeyer flask containing the dissolved salt. Potassium chromate here
served as the indicator for the argentometric determination of chloride ions by reacting to the silver ions
to form a brick-red silver chromate precipitate in the equivalence point region.
Mohr method is the method which uses chromate ions as an indicator in the titration of chloride
ions with a silver nitrate standard solution. After all the chloride has been precipitated as white silver
chloride, the first excess of titrant results in the formation of a silver chromate precipitate, which signals
the endpoint. The reactions involved here are:
Titration reaction
𝐴𝑔+ + 𝐢𝑙 − ↔ 𝐴𝑔𝐢𝑙(𝑠)
Indicator reaction
2𝐴𝑔+ + πΆπ‘Ÿπ‘‚4 2− ↔ 𝐴𝑔2 πΆπ‘Ÿπ‘‚4 (𝑠)
An indicator blank was run first. A small amount of calcium chromate was dissolved in 100 ml
water added with 2 ml potassium chromate indicator. Based on records, the average volume of silver
nitrate used for the 3 trials was 0.47 ml.
From the recorded data, we get the molarity of AgNo3 for trial 1 to be 0.1105, 0.1010 for trial 2,
0.0885 for trial 3 with an average of 0.1000 and a deviation of 0.110. Furthermore, based on the amount
of salt used per trial and the corrected volume of silver nitrate solution and also taking into
consideration the percent purity of the primary standard used, we get titer NaCl for trial 1 to be 6.457,
5.905 for trial 2, and 5.174 for trial 3. The Average Titer NaCl is 5.841 with a deviation of 0.6436.
Another set up was then prepared for the determination of chloride in the sample. From the
dried sample, at least 0.15 g to 0.20 g was measured for the three samples. Each was then dissolved in
50 ml water and a 2 ml 0.1 M potassium chromate solution was then added. Using the data recorded,
the total calculated chloride in the sample for trial 1 was 42.08, for trial 2 it was 49.10, and 42.11 for trial
3. The percent chloride for trial 1 was 26.35%, for trial 2 was 29.51%, and for trial 3 it was 26.87%.
V. CONCLUSION AND/OR RECOMMENDATION(S)
The solution of sodium chloride with the potassium chromate as the color indicator, when
titrated to the silver nitrate solution forms silver chloride and shifts from a yellow solution to a reddish
brown. This color change and clump formation of silver chloride is a sign that the end point is near.
Indicator blank is used to correct the volume of silver nitrate used. Also, the average volume of silver
nitrate calculated in the running of indicator blank also helped in calculating the milligrams of chloride
and the percent chloride used in each trial.
The use of Mohr method in this experiment is important in that it helped in our understanding
of the chemical reactions that occur during each titration process.
VI. REFERENCES
ο‚·
Skoog, Douglas A., et al. Fundamentals of Analytical Chemistry. 8th ed. Singapore: Thomson
Learning Asia, 2004
ο‚·
Korkmaz, Deniz. Precipitation Titration: Determination of Chloride by the Mohr Method.
<http://academic.brooklyn.cuny.edu/esl/gonsalves/tutorials/Writing_a_Lab_Report/xPrecipitati
on%20Titration%20edited%203.pdf>
ο‚·
University of Canterbury. Determination of Chloride by Precipitation Titration with Silver Nitrate
- Mohr's Method. < http://www.outreach.canterbury.ac.nz/chemistry/chloride_mohr.shtml>
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