lab four

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Nicole Lucas
Experiment Four: Comparison of Two Chloride Determinations
Introduction:
The purpose of this lab is to analyze an unknown chloride sample by using two different “wet”
chemical techniques. We will use volumetric and gravimetric determination. For the volumetric
determination, the Fajans method will be used when performing the precipitation titration.
Procedure:
Volumetric Chloride Determination:



Weigh about 4.5 g of AgNO3. Dissolve in 100 mL of distilled water. Transfer the solution
to a 250 mL volumetric flask, by using three 10 mL aliquots of distilled water, and dilute
followed by thorough mixing.
Standardize the AgNO3 using oven dried NaCl. Weigh enough NaCl for a 25 mL
titration. Add 0.03 g of dextrin and five drops of dichlorofluorescein indicator. Titrate the
solution to a pink endpoint.
In a 125 mL Erlenmeyer flask, weigh 0.1 g of the unknown chloride. Dissolve each of the
three samples in 50 mL of water. Add 0.03 g of dextrin and five drops of
dichlorofluorescein indicator. Titrate with silver nitrate solution to a pink endpoint.
Gravimetric Chloride Determination:





Prepare 3 Gooch crucibles by cleaning with an acid solution and then a basic solution.
Follow these washes with distilled water, then drying for at least one hour in a 120 °C
oven. Let the crucibles cool and fit each crucible with a glass fiber filter.
Weigh 0.2 g of the unknown chloride. Dissolve each of the three samples in a 400 mL
beaker with 150 mL of distilled water. Add 1 mL of 6M HNO3.
Add enough silver nitrate solution to completely precipitate the unknown chloride. Heat
the solution on a hot plate without boiling and frequent stirring until the supernatant is
clear.
Vacuum filter the samples through the weighed Gooch crucibles. Use warm 0.1M HNO3
to wash the precipitate. Rinse the precipitate with distilled water.
Dry the crucibles and precipitate in a 120 °C oven for at least two hours before final
weighing.
Chemical Equations:
AgNO3 + Cl− → AgCl(s) + NO−
3
AgCl(s) → Ag + + Cl−
Data:
Volumetric Determination (Mass of AgNO3:4.5084g):
Sample 1
Sample 2
Sample 3
Unknown Cl Dextrin
(g)
(g)
0.1011
0.0334
0.1085
0.0333
0.1062
0.0303
AgNO3(mL) Cl- (g)
% Cl
15.48
16.09
15.89
57.57
55.66
56.21
0.0582
0.0604
0.0597
Mass of NaCl used in titration : 0.1518 g
Mass Dextrin - 0.0324 g
Gravimetric Determination:
Sample 1
Sample 2
Sample 3
Initial Crucible Mass 23.3684
22.6249
23.0140
(g)
Final Crucible Mass 23.8323
23.1024
23.4857
(g)
Product Mass (g)
0.4639
0.4775
0.4717
Unknown Chloride 0.2067
0.2092
0.2066
(g)
AgNO3 (mL)
35.00
35.00
35.00
Cl- mass (g)
0.114
0.118
0.117
% Cl
55.15
56.40
56.63
Amount of silver nitrate to completely precipitate chloride = 31.9
mL
Volumetric %
Gravimetric %
Δ Percentage
Mean
Standard Deviation
t calc
Sample 1
57.57
55.15
2.42
1.19
1.07
1.92
Sample 2
55.66
56.40
0.74
Sample 3
56.210
56.63
0.42
Calculations:
Volumetric Analysis:
Mass NaCl:
Finding grams of Chloride:
Percent of Chloride:
mass chloride
∗ 100 = percent chloride
mass unknown
0.0597g Cl
0.1062g Unknown
∗ 100 = 56.21% Cl-
Gravimetric Analysis:
Mass of AgCl:
Final Crucible Mass – Initial crucible Mass = Mass of AgCl
23.8323 g – 23.3684 g = 0.4639 g AgCl
Volume of AgNO3 solution to be used:
g Unknown * %Cl- = gCl
0.2 g Unknown * 0.5621 Cl- = 0.112 g Cl
Mass of Chloride:
Percent of Chloride:
% ClAverage:
X1+X2+…+Xn / n
2.42+0.74+0.42 / 3 = 1.19
Standard Deviation:
∑(Xi − ̅
X)2
√
n−1
(2.42−1.19)2 +(0.74−1.19)2 +(0.42−1.19)2
√
2
tcalc :
̅|
|X
∗ √n
s
|1.19|
1.07
∗ √3 = 1.92
= 1.07
Conclusion:
This experiment was designed to show the consistency between two different methods applied to
the same sample of unknown chloride. After comparing our results with the student’s t, the
methods were compatible within the 90% confidence interval. Our tcalc, 1.92, was less than the
given value of 2.920. The 90% confidence interval is a large area that the data could be
distributed under. Our value was inbetween the 50% and 90% confidence interval. If more than
three samples were conducted, a more accurate value for the tcalc could have been determined.
We could know then if our value falls more towards the 50% confidence interval or the 90%
confidence interval. The gravimetric determination was the most accurate method for this
experiment. The chloride masses were significantly larger and more precise for the gravimetric
determination than the volumetric determination. We performed an indirect gravimetric analysis
experiment and a direct titration for the volumetric portion.
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