CONTENTS
CONTENTS
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Abstract
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2
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Introduction
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3
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Literature review
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5
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Objectives
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6
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Methodology
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7
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Results
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9
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Discussion
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10
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Conclusion and recommendation
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13
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Reference
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14
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Appendix
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15
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Abstract
This experiment was about gravimetric
determination of chloride. The experiment was carried out to understand the
concept of gravimetric analysis from the determination of amount of analyte
precipitate, silver chloride. The percentage of silver chloride was also
predicted.
In this experiment, silver chloride
was precipitated from the reaction of silver nitrate and sodium chloride. The
precipitate was then washed and filtered, followed by the drying and weighing
processes to obtain the weight of the precipitate itself. With the known
components of the silver chloride precipitate, the weight of chloride could be
calculated, and thus the percentage of chloride in the sample of sodium
chloride could be calculated. This prediction was based on the assumption that
all chlorides in the sample had precipitated to form silver chloride which was
then weighed accurately.
It was important to ensure that all
the chlorides present had reacted in forming the precipitate. Thus, the settled
precipitate was tested with more silver nitrate to ensure complete
precipitation. Stirring helped to prevent bumping of the solution during
heating and the danger of loss of precipitate. However, precipitate might not
be completely dry because it was only heated in the oven for one hour. This
caused the weight of silver chloride obtained to be higher than the predicted
value because water was present in the precipitate.
Introduction
Gravimetric analysis, by definition,
includes all methods of analysis in which the final stage of the analysis involves
weighing. Often, its purpose is to determine the mass or quantity of an analyte
in the original sample when the weight and quantity of the reacted sample is
known.
Gravimetric analysis is one of the
most accurate and precise methods of macro quantitative analysis. This is
because it involves simple steps and usually simple equipment, therefore giving
less room for instrumental errors. The calculation involved does not involve
complicated calculation series which affects the accuracy of the result. This explains
why gravimetric analysis is the method to obtain the atomic masses of many
elements up to six figure accuracy.
Gravimetric analysis can be divided
into two categories, namely precipitation and volatilization, both to produce a
solid state of the analyte. In the process of gravimetric analysis by
precipitation, the analyte is selectively converted to an insoluble form, with
the reaction with suitable reagent. Filtration will then be carried out to
separate the precipitate from the solution or mixture. The separated
precipitate is dried or ignited, possibly to another form, and is accurately
weighed. From the weight of the precipitate and knowledge of its chemical
composition, the weight of anlyte can be calculated in the desired form.
While gravimetric analysis provides
high accuracy and low probability of errors, it is often time consuming,
requires considerable attention to details and is limited to single element or
limited group of elements at a time.
Literature review
Generally, gravimetric analysis is
the method in analytical chemistry, where weight is involved in the analyses.
This can be explained by the term gravimetric, where weight is the effect of
gravity.
Since weighing is the main process in
gravimetric analysis, it is important to present the compound of analyte in an
insoluble form, or solid form. In order to do so, the product is often produced
by a reaction in aqueous solution, using suitable reagent. The product can then
be extraction by either the precipitation method or the volatilization method.
Only pure compound of the product (without the presence of water) should be
weighed to contribute to an accurate analysis. Gravimetric analysis works when
the relative atomic masses of the other components in the sample and analyte
are known. This is because the related information is vital for the calculation
for gravimetric analysis.
In the calculation of gravimetric
analysis, number of moles of precipitate and the required substance is
calculated. This calculation, with the information obtained from the process of
weighing, will then lead to calculations of further information required,
including the most commonly sought percentage of substance in the sample.
In a successful gravimetric analysis,
the sought substance must be completely isolated from the remainder of the
sample. This can be achieved during a successful reaction producing the
insoluble compound, where the sought substance has reacted completely with
another reactant by the fission with its remainder in the sample.
To achieve this goal, precipitates
with very low solubilities (Ksp is very small) are selected and an
excess of the precipitating reagent is added. In addition, the weighed
precipitate must be a pure substance of known chemical composition, and the
precipitate must be easily filtered.
The mechanism for precipitation is
divided into two, namely nucleation and growth. In nucleation, several ions of
the precipitate come together to form a microsize particle called the nucleus;
whereas in growth, the particle grows with the addition of ions of the
precipitate until the precipitate comes to equilibrium. Since the sought
substance must be completely isolated from the remainder in its sample as
mentioned earlier, nucleation is the mechanism used in precipitation for
gravimetric analysis.
Drying or heating the precipitate is
not always the best way to obtain pure weight of the compound containing the
sought substance. In some other cases, it may be easier to remove the compound,
or often called the analyte, by vaporization. The analyte might be
collected—perhaps in a cryogenic trap or on some absorbent material such as
activated carbon -- and measured directly. Or, the sample can be weighed before
and after it is dried; the difference between the two masses gives the mass of
analyte lost. This is especially useful in determining the water content of
complex materials such as foodstuffs.
An example of the application of
gravimetric analysis in industry is the analysis of sulphate in ore. A chunk of
ore is treated with concentrated nitric acid and potassium chlorate to convert
all of the sulfur to sulfate (SO42-). The nitrate and
chlorate are removed by treating the solution with concentrated HCl. The
sulfate is precipitated with barium (Ba2+) and weighed as BaSO4.
The advantages of gravimetric
analysis are significant if methods are followed carefully. Most importantly,
it provides an exceedingly precise analysis. In fact, gravimetric analysis was
used to determine the atomic masses of many elements to six figure accuracy.
This method of analysis also provides very little room for instrumental error
and does not require a series of standards for calculation of an unknown. Also,
methods often do not require expensive equipment. Gravimetric analysis, due to
its high degree of accuracy, when performed correctly, can also be used to
calibrate other instruments.
However, gravimetric analysis usually
only provides for the analysis of a single element, or a limited group of
elements, at a time. Comparing modern dynamic flash combustion coupled with gas
chromatography with traditional combustion analysis will show that the former
is both faster and allows for simultaneous determination of multiple elements
while traditional determination allowed only for the determination of carbon
and hydrogen. Methods are often convoluted and a slight mis-step in a procedure
can often mean disaster for the analysis (colloid formation in precipitation
gravimetry, for example). Compare this with hardy methods such as spectrophotometry
and one will find that analysis by these methods is much more efficient.
Objectives
1.
To understand the concept of gravimetric
analysis.
The
analysis of chloride in silver chloride, AgCl is carried out, mainly by
weighing and calculation.
2.
To determine amount of analyte precipitate.
The
amount of analyte precipitate, silver chloride is determined by its weight.
3.
To predict the percentage of analyte
precipitate.
Percentage
of analyte precipitate by its weight can be calculated by obtaining the weight
of the analyte divided by the weight of its sample compound.
Methodology
Reagent s and Equipment
Concentrated HNO3,
0.1M AgNO3, solid NaCl, burette, 400ml beaker, stirring rod, filter
paper, petri dish, watch glass, magnetic stirrer, hot plate, weighing bottle.
Methods
Precipitation
1. A sample solid sodium chloride of 0.3g is
weighed into 400ml beaker.
2. The sample is dissolved in distilled water and
diluted to 150ml. About 0.5ml of concentrated nitric acid is added.
3. The millimoles of 0.1M silver nitrate required
to precipitate the sample is calculated.
4. The sample solution is heated and silver
nitrate is added slowly until an excess of 10% over the calculated amount, with
constant stirring, to coagulate the silver chloride.
5. The stirring helps prevent bumping of the
solution during heating and the danger of loss of precipitate.
6. The precipitate is let to settle and complete
precipitation is tested by carefully adding a few drops of silver nitrate to
the clear supernatant liquid. If more precipitate or cloudiness appears, a few
more milliliters of silver nitrate solution are added, stirred well, heated,
let to settle, and tested again until precipitation complete.
7. The covered beaker with its content are let to
stand in the desk (protected from the light) for 10 minutes.
Filtration
and washing of the precipitate
8. The solution is decanted by pouring it down a stirring
rod. The precipitate is disturbed as little as possible.
9. 25ml of the wash solution (nitric acid) is
added to the precipitate in the beaker. The mixture is stirred well and the
precipitate is let to settle. The solution is decanted through a weighed filter
paper. Then, the precipitate is brought on the filter paper and put into a
weighed petri dish. Small portion of the wash solution is used for transfer.
Drying and
weighing of the precipitate
10. The petri dish containing the precipitate is
placed in the oven for one hour at 40-50
̊C.
11. The petri dish is cooled in the desiccator and
weighed.
Results
Object
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mass (g)
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Filter paper
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12.0
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Petri dish
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29.2
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Petri dish + filter paper + precipitate
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42.1
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precipitate
|
0.9
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Mass of
precipitate = 42.1 – 29.2 – 12.0
= 0.9 g
Discussion
The balanced
chemical equation of the reaction between sodium chloride and silver nitrate is
NaCl + AgNO3àAgCl
+ NaNO3
The mass of NaCl
used in the experiment (procedure 1) is 0.3g.
The number of
moles of NaCl can therefore be calculated as shown below.
Number of moles
of NaCl = 0.3g/(23.0+35.5)g mol-1
=
mol
mol
From the balanced
chemical equation, it is clearly shown that the ratio of NaCl:AgNO3
is 1:1. Therefore, the number of moles of AgNO3required for the
complete reaction will also be mol.
mol.
To calculate the
volume of the required AgNO3:
mol = 
, where v is volume in cm3.mol = 0.1 mol dm-3 x 
V = 51.3 cm3
To ensure a
complete precipitation of silver chloride, an excess of 10% silver nitrate was
added into the aqueous solution of the sodium nitrate solution.
10% if silver
nitrate = 51.3 cm3 x 10%
= 5.1 cm3
thetotal volume of 0.1M AgNO3 required = 51.3 cm3 +
5.1 cm3
=56.4ml
From the
equation, the predicted number of moles of AgCl precipitated is the same as
that of NaCl used, which is mol.
mol.
Predicted weight of AgCl precipitate
= (107.9 + 35.5)g mol-1 x mol
mol
= 0.735g
From the result obtained in this experiment,
the weight of AgCl is 0.9g.
This result is greater than the predicted
result because the precipitate weighed contained some othercomponents other
than pure AgCl, especially water. This is because the heating and drying
processes were only carried out for one hour. Thus, there might be water
present in the precipitate, and increasing the weight of the precipitate
measured.
After
the precipitate had settled, the beaker containing it was placed in a place
protected from the sunlight. This is because silver chloride is light-sensitive
or photo sensitive. As a result of sunlight exposure, decomposition occurs and
turns the precipitate into silver and chlorine, and the silver remains
colloidally dispersed in the silver chloride and thereby imparts a purple
colour to it. The decomposition by light is only superficial, and is negligible
unless the precipitate is exposed to direct sunlight and is stirred frequently.
Hence the solution was placed in a covered beaker to ensure minimum exposure to
the sunlight.
Questions:
Explain
the function of wash solution (HNO3) during the washing step.
The
aqueous solution of the chloride is acidified with dilute nitric acid in order
to prevent the precipitation of other silver salts, such as the phosphate and
carbonate, which night form in neutral solution, and also to produce a more
readily filterable precipitate.
Calculate the percentage
of chloride in the sample.
In
AgCl,
the
ratio of Ag:Cl in term of mass = 107.9 : 35.5
from
the result:
let
mass of Cl be m, thus the mass of Ag is 0.9g – m
107.9 : 35.5 = (0.9g – m)
: m
107.9m = 31.95g – 35.5m
143.4m = 31.95 g
m = 0.223 g
the mass of the sample =
0.3g
percentage of chloride in
the sample = 0.223g/0.3g x 100%
=52.3%
How to obtain large
precipitate during the experiment?
During the process in the experiment,
when sodium chloride is reacted with silver nitrate, the precipitate of silver
chloride was initially colloidal. It was then turned into large
precipitate by heating the solution and stirring the suspension vigorously,
until the liquid becomes almost clear.
Conclusion and recommendation
Conclusion
From the result
of this experiment, the mass of analyte precipitate is 0.9g, and the percentage by mass of chloride in sodium chloride is 52.3%.
Recommendation
During the
heating of the precipitate, stirring must be done continuously to prevent
bumping of the solution. This also ensures that there will be no loss of
precipitate, and large precipitate can be obtained.
The precipitate
must let to settle before testing for complete precipitation. The testing for
complete precipitation must be observed very carefully, because even very
little cloudiness indicates an incomplete reaction or precipitation. This might
affect the result of mass obtained.
The beaker
containing the precipitate must be protected from sunlight because the
precipitate is photosensitive. The time for it to settle should be prolonged to
one hour.
During the
transfer of the solution from one beaker to another, the precipitate should be
disturbed as little as possible.
The heating of
precipitate in the oven should be prolonged to one day. This enables a more
accurate result to be obtained, where water is absent in the precipitate.
References
§ Earnest, Charles M., and Larry Wilson. 2000. Quantitative Analysis: Gravimetric, Volumetric & Instrumental
Analysis, 4th edition. Loudonville, OH:
Mohican Textbook Pub. Co.
§ Harris, Daniel C. 2007. Quantitative
Chemical Analysis. New York: W.H.
Freeman and Co.
§ Hawkins, M.D. 1970. Calculations
in Volumetric and Gravimetric Analysis. London: Butterworths.
§ Jones, Loretta, and Peter Atkins. 2000. Chemistry: Molecules, Matter and Change.
Gordonsville, VA: W. H. Freeman.
§ D. A. Skoog, D. M. West,
F. J. Holler, and S. R. Crouch Analytical
Chemistry: An Introduction, 7th edt.
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