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8/13/2019 Colligative Property - Elevation in boiling point
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SUNRISE ENGLISH PRIVATE SCHOOL
CHEMISTRY PROJECT
Colligative Property:
Elevation of Boiling Point
Name : Avantika Arun
Class : XII F
Exam No. :
Year : 2013-2014
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Acknowledgement
I would like to thank the CBSE, the school Principal Mr.
Thakur Mulchandani, the Vice Principal Mrs. Sheela P. John &
my parents for being a constant source of support and
information. I also take this opportunity to express my sincere
gratitude to my Chemistry teacher, Mrs. Mini George and the
lab assistant Mrs. Shemna, for guiding me and imparting a
sound base of knowledge pertaining to this topic which
ensured the successful completion of this project. Above all, Ithank the Almighty for his blessings.
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STUDENT PARTICULARS :
NAME : Avantika Arun
CLASS : XII F
EXAMINATION NUMBER :
ACADEMIC YEAR : 2013-2014
SCHOOL : Sunrise English Private School
NAME OF TEACHER : Mrs. Mini George
Signature
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COLLIGATIVE PROPERTY ELEVATION IN BOILING
POINT
OBJECTIVE
To study the effect of addition of a non-volatile solute to a
volatile solvent and also to demonstrate that elevation in
boiling point depends upon the relative number of moles of the
solute and solvent but doesnt depend on the nature of the
solute.
APPARATUS REQUIRED
Bunsen burner, tripod stand, wire mesh, 250ml flask, glass
stirrer, thermometer, tap water, solutes.
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THEORY
COLLIGATIVE PROPERTIES
Colligative properties are those properties of dilute solutions
which depend entirely on the number of moles of solute
contained in the solution and not on the nature of the solute. It
means that two solutions having different components but
same mole-fraction of solute can have identical colligative
properties. Some of the colligative properties are mentioned
below:
I. Relative lowering of vapour pressure.II. Elevation in boiling point.
III. Depression in freezing point.IV. Osmotic pressure.RELATIVE LOWERING OF VAPOUR PRESSURE
For an ideal solution, the relative lowering of vapour pressure
is equal to the mole fraction of the solute. In a solution , a part
of the surface of the liquid is occupied by the solute particles ,
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as such the evaporation of liquid takes place from a lesser
liquid area and hence the liquid will now have a lesser vapour
pressure as less liquid will change into vapours.
Suppose a solution with two components A which isvolatile
and B which is non-volatile. The vapour pressure is given as
Or = K. where K is the constant of proportionality
For pure liquid,
= 1, then K =
, which is the vapour pressure of pure
solvent.
Therefore,
=.
Similarly, if B is also a volatile liquid, the partial vapour
pressure of B, is proportional to its mole fraction. Hence,
, =.
. (1)
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, = .
. (2)
This relationship is known as Raoults law. It states that for a
given solution of two or more miscible volatile liquids, the
vapour pressure of each component at a particular temperature
is directly proportional to its mole fraction.
From equation (1),
= .
Now, suppose B is a non-volatile solute, then
+ = 1, = 1 -
= - .
b
where,
is the relative lowering in the vapour pressure.
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ELEVATION IN BOILING POINT
The boiling point of a liquid maybe defined as the temperature
at which its vapour pressure becomes equal to the atmospheric
pressure. The normal boiling point of pure water is 373 K. The
vapour pressure of the solvent in the solution is lowered due to
addition of non-volatile solute which leads to an elevation in
the boiling point of the solution as now the solution needs
more heating to make its vapour pressure equal to the
atmospheric pressure. This effect has been illustrated in the
vapour pressure curve below:
The curves AB and CD are the vapour pressure curves for the
pure liquid solvent and the solution respectively. At the
temperature T, the vapour pressure of the pure solvent
becomes equal to the atmospheric pressure and T is the
boiling point of the solvent. But the vapour pressure of the
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solution at T is P which is less than atmospheric pressure
and therefore it is needed to heat the solvent to a higher
temperature say in order that the vapour pressure becomes
equal to the atmospheric pressure. Thus is the boiling point
of the solution .Thus it is clear that the solution has higher
boiling point than the pure solvent or . Evidently - is the
elevation in boiling point. Since its magnitude is determined bythe vapour pressure lowering the elevation in boiling point is
also proportional to solute concentration.
(1)
(2)
From (1) and (2),
= K.
= K
+
For dilute solutions,
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=K
= K
= . m [ where m =
]
Where, is the molal elevation constant or molal
ebullioscopic constant.
It is quite clear from the above discussion that we can calculate
molecular mass of solute by measuring the elevation in boiling
point of a solution and elevation in boiling point is a colligative
property.
DEPRESSION IN FREEZING POINT
Freezing point of a substance is the temperature at which solidand liquid phases of the substance co-exist. It is also defined as
the temperature at which liquid as well as solid phases have
some vapour pressure. The freezing point of pure water is
273K. If a non-volatile solute is dissolved in a pure solvent, the
solvent vapour pressure in the solution is depressed which
results in lowering of freezing point. K is the molal depression
constant or cryoscopic constant. Thus, it is clear from the
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discussion that depression in freezing point of solution is a
colligative property.
OSMOTIC PRESSURE
The passage of solvent from pure solvent or solution of lower
concentration to solution of higher concentration is called
osmosis.
Osmotic pressure may be defined as the excess pressure which
is to be applied to the solution side to prevent the passage of
solvent into it through a semi-permeable membrane. Solutions
having same osmotic pressure are called isotonic. The solution
having higher osmotic pressure than a given solution is called
hypertonic and if it has lesser osmotic pressure, it is called
hypotonic. Osmotic pressure is also a colligative property.
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PROCEDURE :
1.Set up the apparatus using a 250ml beaker containing200ml of the experimental solution.
2.Put the beaker on a tripod stand with a wire mesh and usea Bunsen burner to heat the solution
3.A celestial thermometer calibrated up to 110C isimmersed in the solution in the beaker with the help of a
clamp stand.
4.The initial temperature taken before starting theexperiment was considered as the room temperature.
5.At first, find the boiling point of tap water. Thistemperature is taken as the standard boiling point of the
solution.
6.Now, prepare three different concentrations of NaCl andboil 250ml of each one by one in the beaker.
7.Take the readings of the temperature after every 20seconds
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8.After 90 seconds, take the readings after every 10 secondsin order to easily find out the concurrent result.
9.Repeat the procedure similarly for different concentrationsof oxalic acid and take the observations accordingly.
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OBSERVATION
RESULT
On increasing the concentration, the boiling point of NaCl and
oxalic acid increases.
CONCLUSION
The first set of graphs were plotted for temperature of the
solution versus time. In the graph for 1M NaCl, there is slow
rise in the temperature in the first 60 seconds of the
experiment. After that the temperature rises rapidly till 110
seconds. A peak is obtained at 120 seconds. For 2 M NaCl, the
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initial slow rise is almost same as for the 1M graph but the
peak is obtained much faster in 100th second. The boiling
point is obtained by finding the mean temperature. For 3M
NaCl the peak is obtained almost faster at 80 seconds.
In the graph for 1M oxalic acid, a slow and steady rise is seen,
80 seconds after which the graph shoots up. The peak is
obtained at 100 seconds after which the temperature remainsalmost constant. For 2 M oxalic acid, the peak is at 90 seconds
and then for 3M the peak is at 80 seconds. So it is seen that the
time required to attain the peak becomes lesser. From the two
graphs, it is evident that when the concentration of the
solution is increased from 1 M to 3 M, in both cases there is
rise in boiling point.
The increase in temperature in case of NaCl is 97C, 98C and
100.2C. The first two readings are almost the same but for 3
M, the reading differs. This difference can be attributed to
experimental errors as experiment was not conducted in
controlled laboratory conditions.
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Also, the two experiments were not conducted simultaneously
and due to non-availability of distilled water, tap water was also
used. Moreover, due to prolonged heating, some of the solution
evaporates bringing about a change in the actual
concentrations. So it can be suggested that increase in boiling
point is dependent only on the number of moles of solute and
not on the nature of the solute whether it is NaCl or oxalic
acid.
It is proved that when a non-volatile solute is added to a
volatile solvent, the boiling point of the solvent increases. Also,
this increase in boiling point is not dependent on the nature of
the solute but depends only on the number of moles of the
solute. Thus, this elevation in boiling point is a colligative
property.
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Bibliography
Franks SENIOR SECONDARY CHEMISTRY PracticalManual, Frank Bros. & Co., 2012 edition.