Experiment 4: Comparison of Two Chloride Determinations
Andrew Mullenax
Objective:
The objective of this lab is to allow the comparison of two different techniques for the analysis
of an unknown chloride sample.
Procedure:
Volumetric Determination:





Accurately weigh about 4g of AgNO3 and dissolve in 100mL of DI water.
Transfer to a 200mL volumetric flask and dilute to the mark and mix.
Accurately weigh about 0.1g of unknown chloride into 125mL Erlenmeyer flasks.
Dissolve each of the 3 samples in 50mL of water and add 0.03g of dextrin and 5 drops of
dichlorofluorescein indicator.
Titrate with AgNO3 solution to a pink end point.
Gravimetric Determination:
 Prepare 3 gooch crucibles by cleaning with an acid solution(approx. 0.1M HNO3) and
then a basic solution (approx. 0.1M NH3).
 Follow these washes with DI water and then drying for at least one hour in a 120°C oven.
 Fit each crucible with a glass fiber filter.
 Dissolve each of the 3 samples (0.2g) in a 400mL beaker with 150mL DI water and add
1mL of 6M HNO3.
 Add enough AgNO3 to completely precipitate the unknown chloride. Heat on a hot plate
without boiling until supernatant is clear.
 Filter the samples through the weighed gooch crucibles. Use warm approx. 0.1M HNO3
to wash precipitate at least 5 times.
 Rinse the precipitate with DI water and dry the crucibles and precipitate in a 120°C oven
for 2 hours before weighing.
Data:
Volumetric Determination:
Mass AgNO3: 4.0464g
Unknown
Sample
Mass(g)
Volume AgNO3
added(mL)
1
2
3
Average
0.1035
0.1002
0.1009
0.1015
14.55
13.65
13.80
14.00
Mass Cl- in
unknown
sample(g)
.0614
.0575
.0582
0.059
Wt% of Cl- in
unknown sample
59.3
57.4
57.7
58.13
Gravimetric Determination:
Initial Mass(g)
Mass Crucible(g)
Mass Filter
Paper(g)
Final Mass(g)
Mass Product(g)
Mass Cl-(g)
% Cl- in
unknown(g)
Sample 1
.2003
21.5746
Sample 2
.2009
23.2910
Sample 3
.2007
23.0347
Average
0.2006
22.6334
.0309
.0303
.0284
0.0299
22.0695
.4690
.1161
23.7860
.4647
.1150
23.5261
.4630
.1147
23.1272
0.4656
0.1153
58.0
57.2
57.1
57.43
Calculations:
Volumetric Determination:
Mass Cl- in unknown:
𝑣𝑜𝑙𝑢𝑚𝑒 𝐴𝑔𝑁𝑂3 𝑢𝑠𝑒𝑑 𝑥 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 𝑜𝑓𝐴𝑔𝑁𝑂3 ×
. 01455𝐿 𝑥 .119𝑀 𝐴𝑔𝑁𝑂3 𝑥
1 𝑚𝑜𝑙 𝐴𝑔+
1 𝑚𝑜𝑙 𝐴𝑔𝐶𝑙
35.5𝑔
×
×
−
1 𝑚𝑜𝑙 𝐴𝑔𝐶𝑙
1 𝑚𝑜𝑙 𝐶𝑙
1 𝑚𝑜𝑙 𝐶𝑙 −
1 𝑚𝑜𝑙 𝐴𝑔+
1 𝑚𝑜𝑙 𝐴𝑔𝐶𝑙
1 𝑚𝑜𝑙 𝐶𝑙 −
35.5𝑔
×
×
×
+
1 𝑚𝑜𝑙 𝐴𝑔𝑁𝑂3
1 𝑚𝑜𝑙 𝐴𝑔
1 𝑚𝑜𝑙 𝐴𝑔𝐶𝑙
1 𝑚𝑜𝑙 𝐶𝑙 −
= .0614𝑔 𝐶𝑙 −
% Cl- in unknown:
𝑀𝑎𝑠𝑠 𝐶𝑙 −
× 100
𝑀𝑎𝑠𝑠 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
. 0614𝑔
× 100 = 59.3%
. 1035𝑔
Gravimetric Determination:
Mass of final product:
(𝑀𝑎𝑠𝑠 𝐶𝑟𝑢𝑐𝑖𝑏𝑙𝑒 + 𝑓𝑖𝑙𝑡𝑒𝑟 𝑝𝑎𝑝𝑒𝑟 + 𝑝𝑟𝑜𝑑𝑢𝑐𝑡) − (𝑀𝑎𝑠𝑠 𝑐𝑟𝑢𝑐𝑖𝑏𝑙𝑒 + 𝑓𝑖𝑙𝑡𝑒𝑟 𝑝𝑎𝑝𝑒𝑟)
22.0695𝑔 − (21.5746𝑔 + .0309𝑔) = .4690𝑔
Mass of Cl- in unknown:
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡
35.5𝑔 𝐶𝑙 −
×
143.37𝑔
1 𝑚𝑜𝑙 𝐶𝑙 −
𝑚𝑜𝑙
. 4690𝑔 𝐴𝑔𝐶𝑙
35.5𝑔 𝐶𝑙 −
×
= .1161𝑔
143.37𝑔/𝑚𝑜𝑙
1 𝑚𝑜𝑙 𝐶𝑙 −
% Cl- in unknown:
𝑀𝑎𝑠𝑠 𝐶𝑙 −
𝑥 100
𝑀𝑎𝑠𝑠 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
. 1161𝑔
× 100 = 58.0%
. 2003𝑔
Student’s t test comparison:
𝑡𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 =
∑𝑠𝑒𝑡 1(𝑥𝑖 − 𝑥̅1 )2 + ∑𝑠𝑒𝑡 2(𝑥𝑗 − 𝑥̅ 2 )2
|𝑥̅1 − 𝑥̅2 |
𝑛1 𝑛2
, 𝑆𝑝𝑜𝑜𝑙𝑒𝑑 = √
√
𝑆𝑝𝑜𝑜𝑙𝑒𝑑
𝑛1 + 𝑛2
𝑛1 + 𝑛2 − 2
𝑆𝑝𝑜𝑜𝑙𝑒𝑑
[(59.3 − 58.13)2 + (57.4 − 58.13)2 + (57.7 − 58.13)2 ] + [(58.0 − 57.43)2 + (57. −57.43)2 + (57.1 − 57.43)2 ]
= √
3+ 3−2
= .80209
𝑡𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 =
|58.13 − 57.43| 3 × 3
√
= 1.0689
. 80209
3+3
𝑡𝑡𝑎𝑏𝑙𝑒 = 4.303
1.0689 < 4.303
Conclusions:
The purpose of this lab was to compare two different methods for the analysis of an unknown
chloride sample. For the volumetric chloride determination the average weight percent of
chloride in the unknown sample was 58.13%. For the gravimetric chloride determination the
average weight percent of chloride in the unknown sample was 57.43%. Given how close the
averages are for both methods we can conclude that the values are fairly accurate.
Post-Lab Questions:


After using student’s t test to compare the results from the two different methods it was
determined that the calculated t value was les than the t value listed in the table. This
means that the results of the two experiments are comparable.
The greatest source of error in this experiment was most likely the weighing of crucibles
after being in the oven. Not allowing them to cool completely before reweighing would
cause us to have a lower final mass.