Colligative Property - Elevation in boiling point

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.

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Colligative Property - Elevation in boiling point