Amy Komarek
Experiment Four
Comparison of Two Chloride Determinations
Introduction:
The point of today’s experiment is to perform the comparison of two “wet” chemical
techniques. The first technique is a volumetric chloride determination. The second technique is a
gravimetric determination. These techniques will give enough data compare the methods for
analyzing an unknown chloride sample statically.
Procedure:
Volumetric Chloride Determination:
1. Weigh about 4.5g of AgNO3 and dissolve in 100mL of distilled water. Transfer the
solution to a 250mL volumetric flask by using three 10mL aliquots of distilled water, and
dilute by mixing.
2. Standardize the AgNO3 using oven dried sodium chloride. Weigh enough sodium
chloride for a 25mL titration. Add 0.03g of dextrin and 5 drops of dichlorofluorescein
indicator. Titrate the solution to pink endpt.
3. Into a 125mL Erlenmeyer flask, weigh 0.1g of the unknown chloride. Dissolve each of
your three samples in 50mL of water. Add 0.03g dextrin and 5 drops of
dichlorofluorescein indicator. Titrate with silver nitrate solution to a pink endpt.
Gravimetric Chloride Determination:
1. Prepare 3 Gooch crucibles by cleaning with an acid solution (0.01M HNO3) and then a
basic solution (0.1M NH3). Follow these washes with distilled water and then drying for
at least an hour in a 120°C oven. Let the crucibles cool and fit each crucible with a glass
fiber filter.
2. Weigh 0.2g of unknown chloride. Dissolve each of the 3 samples in a 400mL beaker with
150mL distilled water. Then add 1mL of 6M HNO3.
3. Add enough silver nitrate solution to completely precipitate the unknown chloride. Heat
the solution on hot plate without boiling and frequent stirring until clear.
4. Vacuum filter the sample through to the weighed Gooch crucibles. Use warm 0.1M
HNO3 to wash the precipitate 5 times. Rinse precipitate with distilled water.
5. Dry the crucibles and precipitate in a 120°C oven for at least 2 hrs 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
AgNO3(mL) Cl- (g)
% Cl
15.48
0.0582
57.57
0.1085
0.0333
16.09
0.0604
55.66
0.1062
0.0303
15.89
0.0597
56.21
Mass of NaCl: 0.1518g
Mass of Dextrin: 0.0324g
Gravimetric Determination:
Sample 1
Initial Crucible Mass 23.3684
(g)
Final Crucible Mass 23.8323
(g)
Product Mass (g)
0.4639
Unknown Chloride 0.2067
(g)
AgNO3 (mL)
35.00
Cl- mass (g)
0.114
% Cl
55.15
Sample 2
22.6249
Sample 3
23.0140
23.1024
23.4857
0.4775
0.2092
0.4717
0.2066
35.00
0.118
56.40
35.00
0.117
56.63
Silver nitrate need to precipitate chloride: 31.9mL
Calculations:
Volumetric Analysis:
Mass NaCl:
Finding grams of Chloride:
Percent of Chloride
mass chloride
∗ 100 = percent chloride
mass unknown
0.0597g
∗ 100 = 56.2%
0.1062g
Gravimetric Analysis:
Mass of AgCl
Final crucible mass − Inital crucible mass = Mass of AgCl
23.8323g − 23.3684g = 0.4639g AgCl
Mass of Chloride
mass AgCl ∗
mol AgCl
mol Cl− molar mass Cl−
∗
∗
= mass of Cl−
molar mass AgCl mol AgCl
mol Cl−
mol AgCl
mol Cl− 35.45g Cl−
0.4639g AgCl ∗
∗
∗
= 0.114g Cl−
143.321g AgCl mol AgCl
mol Cl−
Percent of Chloride
mass Cl−
∗ 100 = percent chloride
mass unknown
0.114𝑔
∗ 100 = 55.15%
0.2067g
Average
X1 + X 2 + X n
= average
n
2.42 + 0.74 + 0.42
= 1.19
3
Standard Deviation
̅) 2
∑(Xi − X
√
= Standard Deviation
n−1
∑(2.42 − 1.19)2 + (0.74 − 1.19)2 + (0.42 − 1.19)2
√
= 1.07
3−1
tcalc
̅|
|X
∗ √n = t calc
s
̅̅̅̅̅̅|
|1.19
∗ √3 = 1.92
1.07
Volumetric %
Gravimetric %
Δ Percentage
Mean (all samples )
StandardDeviation
(all samples)
Tcalc (all samples)
Sample 1
57.57
55.15
2.42
1.19
1.07
Sample 2
55.66
56.40
0.74
Sample 3
56.210
56.63
0.42
1.92
Discussion:
1. After calculating the student t we were able to see that the two separate methods were
compatible within the 90% confidence interval. We calculate a t, 1.92; this is less than the
value given which was 2.92. Our tcalc was found between the 50% and the 90%
confidence interval. We can determine that the gravimetric method was the most accurate
method because our t value was closer to the 50% confidence interval. We also found that
our chloride masses were more precise through the gravimetric method. If we wanted a
more accurate tcalc then we would need to conduct several more samples.
2. We performed an indirect gravimetric analysis.
3. We performed a direct titration.
Conclusion:
There are several procedural changes that could be made to improve the lab experiment.
One change could be to use more than three samples. Using more samples will help the data not
be affected dramatically compared to when only three samples are used. Using more samples
will also help the data to be more precise and our tcalc would also be a lot more accurate.