Tim Abell
Experiment 4: Comparison of Two Chloride Determinations
Purpose: To compare two different “wet” chemical techniques for the analysis of an unknown chloride
sample. Then compare the two methods statistically.
Procedure:
Method 1:
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Weigh about 4g AgNO3 put in a 250mL volumetric flask and dilute with water to the line
Weigh about .1g of unknown into a 250mL flask (3 times)
Dissolve in 50mL of water, add .03g of dextrin, and 5 drops of dichlorofluorescein
Titrate with silver nitrate until pink (3 times)
Method 2:
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Prepare 3 crucibles by washing them in acid, base, and then water
Dissolve .2g of unknown in a 400mL beaker with 150mL of water, then add 1mL of 6MHNO3 (3
times)
Add enough silver nitrate to fully precipitate the unknown chloride, then heat without boiling
Filter through a gooch crucible
Wash with warm .1M HNO3 (5 times), then with water
Dry crucible and precipitate for at least 2 hours in an oven, then weigh the crucible and product
Reactions:
AgNO3 + Cl-→ AgCl(s) + NO3AgCl(s)→ Ag+ + ClData:
.0946M AgNO3
Method 1:
Trial
Unknown Mass (g)
1
2
3
.1108
.1305
.1331
AgNO3 Volume
(mL)
18.69
20.70
24.47
Cl Mass (g)
% Cl in Unknown
.0628
.0695
.0822
56.68%
53.26%
61.76%
Avg: 57.23%
Method 2:
Trial
Unknown Mass
(g)
Crucible, Filter
Paper Mass (g)
1
2
3
.2760
.2379
.2605
17.2915
16.0846
17.6703
Crucible, Filter
Paper, Product
Mass (g)
17.6665
16.4437
18.0022
Product Mass
(g)
Cl Mass
(g)
% Cl in
Unknown
.3750
.3591
.3319
.09283
.08890
.08216
33.63%
37.37%
31.54%
Avg: 34.18%
Comparison:
Trial
1
2
3
Method 1 (%)
56.68
53.26
61.76
Method 2 (%)
33.63
37.37
31.54
Difference
-23.05
-15.89
-30.22
Avg: -23.05
Standard Devation: 7.17
Tcalc: 5.57
Calculations:
Molarity of AgNO3
Method 1 mass of Cl in unknown
Method 1 % Cl
Method 2 mass Cl in unknown
Method 2 % Cl
M AgNO3= Mass
AgNO3*(1mol/molar mass
AgNO3)*(1/L)
Mass Cl =M AgNO3*L AgNO3*(molar
mass Cl/ 1mol)
%Cl= (mass Cl/mass of
unknown)*100
Mass Cl= mass AgCl*(1mol/molar
mass AgCl)*(molar mass Cl/1mol)
%Cl= (mass Cl/mass of
unknown)*100
4.0180g
*(1mol/169.9g)*(1/.250L)=.0946M
AgNO3
.0946 M * .01869 L* (35.5g/ 1 mol)=
.0628g Cl
(.0628g/.1108g)*100= 56.68%
.3750g AgCl
(1mol/143.4g)*(35.5g/1mol)=
.09283g Cl
(.09283/.2760)*100= 33.63%
Conclusion:
After statically comparing the two methods (volumetric and gravimetric determination) they are
not similar. The tcalc= 5.57 which is greater than the ttable= 4.303 at the 95% confidence level. It isn’t
until the 98% confidence level, ttable= 6.965, that the tcalc falls below the ttable.
By eliminating some of the error the methods may have been similar. One of these errors
includes adding enough silver nitrate to precipitate out all of the chloride in method 2. All of the chloride
was most likely not precipitated out which led to a very low mass percent of chlorine. Due to this error
one cannot confidently say whether or not these methods are similar. In order to determine that the
experiment would have to be done again while attempting to eliminate as much error as possible.