Comparison of Two Chloride Determinations
Purpose:
The comparison of two different “wet” chemical techniques. This will be done
with the analysis of an unknown chloride sample. The first technique will be the
volumetric chloride determination, while the second will be a gravimetric
determination. These Two methods will then be compared statistically.
Procedure:
Volumetric Chloride Determination
Weigh out 4g of AgNO3, place this in 100mL of distilled water. Transfer to a 200mL
volumetric flask. Weigh about 0.1g of unknown chloride; dissolve each in 50mL
distilled water. Add 0.03g of dextrin and 5 drops of dichlorofluorescein indicator.
Then titrate with the silver nitrate solution the endpoint will be pink. Calculate the
percent chloride in sample.
Gravimetric Determination of Chloride
Prepare three Gooch crucibles, standard procedure for cleaning and massing.
Dissolve each of the three samples that should be about 0.2g in a 400mL beaker
with 150mL-distilled water. Add 1mL of 6M HNO3. Add silver nitrate to precipitate
out the unknown chloride. Heat until the supernatant is clear. Filter the sample
through the Gooch crucibles with suction. Use warm 0.1M HNO3 to wash precipitate
5 washes, and then rinse with distilled water.
Reactions:
AgNO3 + Cl- AgCl (s) + NO3AgCl (s)  Ag+ + ClData:
Mass of AgNO3=4.024g
Volumetric Determination
Trial Unknown Chloride
Mass (g)
1
0.1032
2
0.1070
3
0.1023
Dextrin Mass (g)
mL of AgNO3 used for titration
0.0303
0.0303
0.0312
14.80
15.30
17.08
Volumetric Determination Calculated Data
Trial Molarity Moles AgNO3 & Mass Cl- in Precipitate
AgNO3
Cl(g)
1
0.119
0.00176
0.0624
2
3
0.119
0.119
0.00182
0.00203
0.0645
0.0721
Mass % Cl- in Uknown
60.5%
60.2%
70.5%
Gravimetric Determination
Trial
Unk.
AgNO3 Crucibles
Chloride added
(g)
Mass (g) (mL)
1
0.2236
8.5
30.2575
2
0.2284
8.5
30.4228
3
0.2274
8.5
29.5413
Crucible,
Product (AgCl)
Product, &
(g)
Filter Paper (g)
30.3863
0.1288
30.5599
0.1371
29.6668
0.1255
Gravimetric Determination Calculated Data
Trial
Mass Cl- in precip. (g)
1
2
3
0.0318
0.0339
0.0310
Calculations:
Volumetric Determination
Molarity of AgNO3
0.119𝑀 =
1 𝑚𝑜𝑙
4.024𝑔 × 169.873𝑔
. 0148𝐿
Moles of AgNO3 & Cl- Trial 1
0.119𝑀 × 0.01480𝐿 = 0.00176 𝑚𝑜𝑙
Mass of Cl- in Precipitate Trial 1
35.453𝑔𝐶𝑙 −
0.00176𝑚𝑜𝑙𝐶𝑙 − ×
1 𝑚𝑜𝑙 𝐶𝑙 −
= 0.06324𝑔𝐶𝑙 −
Mass Percent of Cl- in Unknown Trial 1
0.0624𝑔 𝐶𝑙 −
× 100% = 60.5% 𝐶𝑙 −
0.1032𝑔 𝑈𝐾
Gravimetric Determination
Mass of AgCl (precipitate) Trial 1
30.3863𝑔 − 30.2575𝑔 = 0.1288𝑔 𝐴𝑔𝐶𝑙
Mass of Cl- in precipitate Trial 1
0.1288𝑔 ×
1 𝑚𝑜𝑙
1 𝑚𝑜𝑙 𝐶𝑙 −
35.453𝑔
×
×
= 0.00318𝑔 𝐶𝑙−
143.321𝑔 1 𝑚𝑜𝑙 𝐴𝑔𝐶𝑙
1 𝑚𝑜𝑙
Mass Percent of Cl- in Unknown Trial 1
0.0318𝑔 𝐶𝑙 −
× 100% = 14.4% 𝐶𝑙 −
0.2236𝑔 𝑈𝐾
Mass % Cl- in unknown
14.4%
14.8%
13.6%
t calc =
Volumetric %
60.5
60.2
70.2
Avg. 63.633
|𝑑̅ |
√n
𝑠
Gravimetric
%
14.4
14.8
13.6
14.266
Difference
%
46.1
45.4
56.6
49.366
*Standard deviation was calculated using excel
49.366
× √6 = 4.434
27.28
ttable for at 95% confidence interval is = 2.776
4.434>2.776 the two determinations are statically different
Conclusion:
When the two methods are compared a tcalc is found to be much greater at
4.434 than the t-table for the 95% confidence interval which is 2.776. This means
that we cannot compare the two different methods for determination of chloride.
The overall volumetric determination went well, while in the gravimetric
determination there was a clear error in the experimental values.
To improve upon this experiment there needs to be more AgNO3 added to the
solution to completely precipitate out the AgCl. This is shown with the large percent
different between the two methods, and the low percent yields of AgCl at an average
of 14.27. It is also shown using the t test; there was a very large difference between
the two methods and the methods are not comparable. Another source of error
could come from the indicator used during the titrations it was difficult to see the
pink change and was easy to go over the endpoint. Using a different indicator may
there may be less error.