COLLIGATIVE PROPERTY: FREEZING POINT DEPRESSION AND MOLAR MASS
Elsbeth Nicole P. Villanueva Rixs G. Itum
BS Chemical Engineering BS Chemical Engineering
Xavier University Xavier University
Abstract Freezing point depression is a The freezing points of solutions are all lower
colligative property observed in solutions that than that of the pure solvent and are directly
results from the introduction of solute molecules proportional to the molality of the solute.
to a solvent.Moles and Molar Mass The mole is
the "counting unit" used by chemists to indicate tf=Tf(solvent)Tf(solution)= Kf m
the number of atoms, ions, molecules, or Where tf is the freezing point
formula units present in a particular chemical
depression, Tf (solution) is the freezing point of
sample. the solution, TfTf (solvent) is the freezing point
The experiment aims to determine the molar of the solvent, Kf is the freezing point depression
mass of an unknown solid of freezing point constant, and m is the molality. The freezing
depression and to determine the identity of an point of the solvent in a solution will be less
unknown solid through molar mass
than the freezing point of the pure solvent
measurements.The average freezing point whether the solution contains a non-volatile
calculated is 78.95082256 .The freezing solute or a volatile solute.
point of pure Naphthalene is being subtracted
with the freezing point of Naphthalene with the However, for simplicity, only non-
unknown solid resulted to the freezing point volatile solutes will be considered here.
depression of 0.4682686721 .The calculated Experimentally, we know that the change in
value of the molar mass of the unknown solid freezing point of the solvent in a solution from
is211.2229513 .Out of the five possible that of the pure solvent is directly proportional
identities of the unknown solid, the molar mass to the molal concentration of the solute:
closest to the experimental value of the molar T = Kfm
mass of unknown solid (211.2229513 ) is the
Where: T is the change in freezing point of
molar mass of Dibromobenzene (235.90)
the solvent, Kf is the molal freezing point
and the percent error obtained is 10.46%. depression constant, and m is the molal
Keywords: Freezing Point Depression, concentration of the solute in the solution. Note
Colligative Property, Naphthalene, Molar that the molal freezing point depression
Mass. constant, Kf, has a specific value depending on
the identity of the solvent. [1]
Moles and Molar Mass The mole is the
I. INTRODUCTION "counting unit" used by chemists to indicate the
number of atoms, ions, molecules, or formula
Freezing point depression is a colligative units present in a particular chemical sample.
property observed in solutions that results from The mole is similar to other counting units that
the introduction of solute molecules to a solvent. you've used before....pair (2), dozen (12), and
gross (144). One mole of a compound contains unknown solid. The average freezing point
Avogadro's number (6.022 x 1023) of molecules calculated is 78.95082256 .
(molecular compound) or formula units (ionic
compound). The molar mass of a compound tells Table 1: Freezing point of naphthalene with
you the mass of 1 mole of that substance. [2] unknown solid
II. EXPERIMENTAL SECTION Trial Freezing point
1 78.83989829 C
15 to 20 grams of naphthalene was added to 2 79.06174743 C
a large test tube that was weighed beforehand.
The naphthalene was then subjected to water Freezing point depression has been
bath at the heat of 85C and a stopper and a determined using the formula = .
thermometer was added at the time the The freezing point of pure Naphthalene is being
naphthalene was melted. At the time the subtracted with the freezing point of
naphthalene was melted immediately place it Naphthalene with the unknown solid resulted to
into wide-mouthed bottle with some paper the freezing point depression
towels at the bottom. Record temperature for of 0.4682686721 .
every 30 seconds. Stir naphthalene. Allow
naphthalene to cool when temperature is The third and last part is the calculation of the
constant. Plot time vs. temperature to get the molar mass of the unknown solid. Given with
freezing point of the pure naphthalene. the cryoscopic constant of Naphthalene, mass of
solute, mass of solvent and the freezing point
depression of the solution, the molar mass of the
For the determination of molar mass of the unknown solid can be calculated. The calculated
unknown solid. Weigh the given unknown solid value of the molar mass of the unknown solid
sample and add it to naphthalene in the test tube. is211.2229513 . Table 2 shows the known
Replace stopper and stir the solution gently until molar masses of the possible identity of the
unknown solid dissolves. Remove test tube in unknown solid.
heat and place it into the wide-mouthed bottle
with paper towels in the bottom. Record Table 2: Known molar masses of possible
temperature for every 30 seconds. Stir and allow identity of the unknown solid
it to cool when temperature is constant. Plot time
vs. temperature to get the freezing point of the Compound Molar mass (g/mol)
unknown solid and naphthalene. Dichlorobenzene 146.998
Dibromobenzene 235.90
Anthracene 178.234
III. RESULTS AND DISCUSSION Benzophenone 186.222
Sulfur 32.0650
The first part of the experiment is the
Identification of the unknown solid is the last
interpolation of the freezing points given with
1 2 2 2
part of the experiment. Out of the five possible
the formula of = .Table 1 shows identities of the unknown solid, the molar mass
1 2
the freezing point of Naphthalene with the closest to the experimental value of the molar
mass of unknown solid (211.2229513 ) is the
1 2 2 2
molar mass of Dibromobenzene (235.90 ) and =
the percent error obtained is 10.46%. 1 2
Pure Napthalene:
At:
IV. ERROR ANALYSIS m1 = -0.1431 b1: 80.04
m2 = -0.1136 b2: 79.912
Some errors that occur in the experiment is that
we failed to immediately transfer the test tube in (0.1431)(79.912) (0.1136)(80.04)
the beaker which causes some inaccuracy of the =
(0.1431) (0.1136)
results in the temperature determination. . The = 79.41909153
inaccuracy of the apparatus such as the
analytical balance is also a factor to the errors in
Napthalene with Unknown solid:
this experiment.
Trial 1:
At:
m1: -4.1488 b1:89.675
V. CONCLUSION
m2: -0.0793 b2:79.047
The main objective of the experiment is to
determine the freezing point of pure
naphthalene. Its approximete freezing point is (4.1488)(79.047) (0.0793)(89.675)
=
78.95 and the average freezing point of the (4.1488) ( 0.0793)
mixture of naphthalene with unknown solid = 78.83989829
is78.95 C.with these we can then identify the
the freezing point depression of the compound Table 1: Freezing point of naphthalene
and it freezing point depression is 0.4683 C. with unknown solid
The closest possible identity of the unknown Trial Freezing point
solid is dibromobenzene. The experimental 1 78.83989829 C
value of the molar is 211.22 g/mol with a 2 79.06174743 C
percent error of 10.46%.
Average of Trial 1 and 2:
REFERENCES 78.83989829 + 79.06174743
=
2
[1] http://hyperphysics.phy- = 78.95082256 C
astr.gsu.edu/hbase/Chemical/meltpt.html#c1
[2]
https://www.uwplatt.edu/~cornettc/Colligative% B. Freezing Point Depression
2520properties.DOC
=
=
= 79.41909153
APPENDICES 78.95082256
= 0.4682686721
A. Interpolation of Freezing points
C. Molar mass of unknown solid
()()
= ( ) 1000
( )()
(6.80 )(0.220 )
= ( ) 1000
(0.4682686721)(15.125)
= 211.2229513
D. Percent errors against known molar
masses
Table 2: Known molar masses of
possible identity of the unknown solid
Molar mass
Compound
(g/mol)
Dichlorobenzene 146.998
Dibromobenzene 235.90
Anthracene 178.234
Benzophenone 186.222
Sulfur 32.0650
% = | |
100
Percent error against Dibromobenzene:
211.2229513 235.90
% = | |
235.90
100
