experiment: gravimetric analysis
TRANSCRIPT
TITLE: The Gravimetric Determination of Chloride
OBJECTIVES OF EXPERIMENT:
After completing the experiment, the students should be able to:
1. List down the important factors which effect the crystallinity/size of precipitate.
2. Describe the steps to be taken to ensure that the precipitates formed are large and easily
filtered and avoid peptization.
3. Carry out the gravimetric experiment confidently.
4. Calculate the theoretical yield of a product.
5. Calculate the percentage yield of a product.
INTRODUCTION:
Gravimetric methods are among the most accurate and precise methods of quantitative
analysis. These advantages are counterbalanced by the fact that they are often time consuming,
require considerable attention to detail and are limited to sample size and concentrations which
yield a weighable quantity on a conventional analytical balance. While many gravimetric
methods have been at least partially replaced by newer, faster techniques, there remain several
situations where the only suitable analytical techniques are gravimetric.
Addition of a solution containing silver ion [usually an aqueous silver nitrate (AgNO3)
solution] to a dissolved sample containing chloride ion will quantitatively precipitate the chloride
ion as solid silver chloride (AgCl), due to the small solubility product constant of silver chloride
(ksp = 1.82 x 10-10). The precipitate may be isolated from solution by filtration, the material
dried, weighed and the percent chloride (% Cl-) calculated from the stoichiometry of the reaction:
Ag+ (aq ) + Cl−(aq ) AgCl(s )
In addition to possible interferences common to all gravimetric methods, the presence of any
other halide will cause high results since they produce a silver halide which is even more
insoluble than silver chloride. Also, silver chloride is light sensitive and excessive
photodecomposition will produce erroneous results, according to the reaction:
AgCl(s ) hv Ag(s ) + 1
2Cl2 (g )
The precipitate becomes violet-purple, due to the presence of finely divided silver metal, and
results will be low. If silver ion (Ag+) is present, in addition to the above, the following reaction
will also occur as a result of the photodecomposition reaction:
3Cl2( g ) + 3H2O( l) + 5Ag+ (aq ) h⃗v 5AgCl(s ) + ClO3−( aq ) + 6H+ (aq )
If this reaction predominates, the results will be high. To minimize such errors, it is
recommended that unnecessary exposure to light be avoided.
Gravimetric analysis, if methods are followed carefully, provides for exceedingly precise
analysis. In fact, gravimetric analysis was used to determine the atomic masses of many elements
to six figure accuracy. Gravimetry provides very little room for instrumental error and does not
require a series of standards for calculation of an unknown. Methods also do not require often
expensive equipment. Gravimetric analysis, due to its high degree of accuracy, when performed
correctly, can also be used to calibrate other instruments in lieu of reference standards.
METHODOLOGY:
RESULT:
Weight of dried NaCl:
Sample 1: 0.3029 g
Sample 2: 0.3001 g
Sample 1 2
Initial reading (cm3) 0.30 1.80
Final reading (cm3) 17.60 17.30
Volume of AgNO3 used(cm3) 17.30 15.50
2 sets of 0.3 g of dried NaCl was
weighed accurately and
were transferred into a 400 cm3 beaker each.
They were diluted with 100 cm3 of distilled
water in which 3 cm3 of 6 M
HNO3 has been added.
Slowly, while stirring, 0.2 M
AgNO3 was added from a burette to the
sample solutions until AgCl was
observed to coagulate. Then, 5 cm3
extra AgNO3 was introduced.
The samples were boiled and
left for 10 minutes.
A few drops of AgNO3 was
added. If more precipitate
formed, 3 cm3 AgNO3 was
added. The completeness of precipitate was
tested again.
Each beaker was covered and
stored in a dark place before
filtration.
Two crucibles were clean and
dried in the oven. The weight of
each of the crucible and a piece of filter
paper was recorded.
The precipitate was washed several times
with 5 cm3 of 6 M HNO3 per liter of distilled water.
The washing must be done
finely and rubber policemen was
used to dislodge any particles at the walls of the
beakers.
The washing was continued until
the filtrates were essentially free
of Ag+ ion.The AgCl was quantitatively
transferred into individual crucibles.
The precipitates were dried at
110 C or at least ⁰1 hour.
The crucibles were stored in the desiccator
while they cooled.
The mass of the crucibles and their contents
were determined.
Sample 1 2
Weight of crucible + filter
paper (g)
74.0715 46.8966
Weight of crucible + filter
paper + precipitate (g)
74.5413 47.3095
Weight of precipitate (g) 0.4698 0.4129
CALCULATION:
A. Theoretical Data
1. Minimum volume of AgNO3
Mr of NaCl = (22.989770 + 35.453) g/mol = 58.44277 g/mol
Mr of AgCl = (107.8682 + 35.453) g/mol = 143.3212 g/mol
Mr of Cl- = 35.453 g/mol
2. Equation: Na+ (aq) + Cl- (aq) → NaCl(aq)
NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
3. n NaCl = Mass NaClMr of NaCl
= 0.3 g
58.44277 g /mol = 0.00513 mol
4. From equation, 1 mol of Cl- ≡ 1 mol of NaCl
Mass of Cl- = n Cl- × Mr Cl
-
= 0.00513 mol × 35.453 g/mol = 0.1819 g
5. Theoretical percentage of Cl- in NaCl = MassCl−¿
Mass of NaCl¿ × 100 %
= 0.1819 g
0.30 g × 100 % = 60.63 %
6. Minimum volume of 0.2 M of AgNO3 required:
1 mol of NaCl ≡ 1 mol of AgNO3
n NaCl = 0.00513 mol
n = MV1000
V = 1000 n
M =
1000(0.00513 mol)0.2 M
= 26.65 cm3
B. Sample 1
1. n AgCl = Mass AgClMr of AgCl
= 0.4698 g
143.3212 g /mol = 0.00328 mol
2. NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Ag+ (aq) + Cl- (aq) → AgCl (s)
1 mol AgCl ≡ 1 mol Cl-
Mass of Cl- = n Cl- × Mr Cl
-
= 0.00328 mol × 35.453 g/mol = 0.1163 g
3. Percentage of Cl- obtained = MassCl−¿
Mass of NaCl¿ 100 % =
0.1163 g0.3029 g
× 100 % = 38.40
%
4. Percentage of error = | Theoretical% Cl−¿−Experimental% of Cl−¿
Theoretical %Cl−¿ ¿¿¿| × 100%
= | 60.63−38.40
60.63| × 100% = 36.67 %
Sample 2
1. n AgCl = Mass AgClMr of AgCl
= 0.4129 g
143.3212 g /mol = 0.00288 mol
2. NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Ag+ (aq) + Cl- (aq) → AgCl (s)
1 mol AgCl ≡ 1 mol Cl-
Mass of Cl- = n Cl- × Mr Cl
-
= 0.00288 mol × 35.453 g/mol = 0.1021 g
3. Percentage of Cl- obtained = MassCl−¿
Mass of NaCl¿ 100 % =
0.1021 g0.3001 g
× 100 % = 34.02
%
4. Percentage of error = | Theoretical% Cl−¿−Experimental% of Cl−¿
Theoretical %Cl−¿ ¿¿¿| × 100%
= | 60.63−34.02
60.63| × 100% = 43.89 %
DISCUSSION:
Gravimetric analysis is used to determine the amount of a substance by finding its mass,
and then using the mass to find the quantity of the substance. One of the most common
techniques of applying gravimetric analysis is obtaining a precipitate from a solution and
removing any impurities from it, in order to find its net mass. Stoichiometry is the study of the
relationships between the quantities of reactants and products involved in chemical reactions,
and is used in many calculations involving molar and mass ratios. Gravimetric stoichiometry is a
combination of gravimetric analysis and stoichiometry, and in short it is the procedure for
calculating the masses of reactants, or products, in a chemical reaction. Gravimetric
stoichiometry is used in this experiment to calculate the theoretical yield of a chemical reaction,
and the percentage yield (what percentage of the amount predicted for a substance was actually
produced).
The objective of this experiment is to obtain percentage yield of chloride ions in a
solution of sodium chloride, using silver nitrate as a reagent. When sodium chloride is added to
silver nitrate, a double displacement reaction occur which results in silver chloride and sodium
nitrate.
The experiment was started by dissolving dried NaCl salt in distilled water and 6 M
HNO3 solution. In this solvation process, acid solution has been added first in distilled water
before adding the salt so that the salt completely dissolving in homogenous dilute acid solution.
The addition of AgNO3 solution to form precipitate of AgCl was carried out slowly and with
continuous stirring to let the reaction completion. If not enough time for the reaction to complete,
a lower yield of the precipitate would occur. More actions were taken so that ‘good’ precipitate
formed; which heating the precipitates solution and let it to digest then carried out the test for
completeness of the precipitation.
As the reaction may involve photodecomposition of AgCl salt, the salt was left to
continue its reaction under darkness by covering its container with aluminium paper and put into
laboratory table which no light could enter. During the filtration process, solution of HNO3 was
used to wash the precipitates thoroughly to decrease the possibility of peptisation of the salts.
The washing process also needed rubber policemen to remove any particles that adhere to the
walls of the container. The washing steps was free of Ag+ ions when no more cloudy solution
seen in the waste solution of the wash.
After the calculation, the percentage yield of AgCl precipitate was 38.40 % in Sample 1
while in Sample 2 was 34.02 % from the theoretical value. The high percentage of errors, for
Sample 1 was 36.67 % and 43.89 % for Sample 2 mainly induced by one step missed which was
incomplete precipitation reaction due to not adding enough precipitating agent(AgNO3).
Minimum amount of the precipitating agent should be used was 26.65 cm3. However in this
experiment, the minimum amount was not reached resulting in low percentage yield of products.
Therefore, the experiment can be improved by avoiding these gravimetric errors that leading to a
lower mass of precipitates. One more thing that was important, always weight the dried
precipitates so the correct value was recorded.
PRECAUTION STEPS:
1. Use AgNO3 carefully and sparingly. It leaves dark stains (metallic silver) on equipment,
skin and laboratory surfaces. If spill occurs wipe up immediately and rinse thoroughly
with plenty of water.
2. Crucible must be dried in the oven first to remove moist in its content before using it to
reduce reading errors when weighing the precipitates.
3. The precipitates must be dried completely before weighing it to avoid obtaining wrong
reading of the precipitates.
CONCLUSION:
1. The crystallinity/size of precipitate is affected by choosing correct reagents with correct
cations. Ag+ ions is most suitable cation to produced large size AgCl precipitates when it
coagulates with Cl- ions.
2. To ensure that the precipitates formed are large and easily filtered and avoid peptisation,
these steps are taken:
- The precipitates was heated and let it digest in enough time
- Washed with volatile reagent such as HNO3 solution to avoid redissolvation of
precipitates
3. The experiment was done confidently by the way of gravimetric analysis.
4. The theoretical yield of Cl- calculated is 60.63 %.
5. The percentage yield of Cl- obtained in Sample 1 was 38.40 % while in Sample 2 was
34.02 %.
6. The percentage of errors in experiment for Sample 1 was 36.67 % and 43.89 % for
Sample 2.
REFERENCES:
1. Gary D.Christian: Analytical Chemistry Sixth Edition, by John Wiley and Sons Inc.
2. Nelson, J., Chemistry:The Central Science, 3rd Edition, Prentice- Hall, 1985.
3. https://vinstan.wikispaces.com/Final+Project+
+Gravimetric+Estimation+of+Chloride+Ions
4. D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch Analytical Chemistry: An
Introduction, 7th ed. Chapter 8, pp. 179-201.
QUESTIONS:
Explain why in gravimetric analysis of chloride ion using ion silver precipitation:
An excess of silver ions is added so that the chloride concentration at equilibrium will be
negligible. If enough silver nitrate solution is not used, the precipitation will be
incomplete, resulting in a substantial error and a low value for the % Cl in the sample.
Nitric acid and the small excess of silver nitrate promote coagulation by providing a
moderately high electrolyte concentration. Nitric acid in the wash solution maintains the