Lab Report 3

Molecular mass determination by depression of the

freezing point.
Objective:
In this experiment, we determined the freezing point of cyclohexane and the freezing point of
a solution containing a weighed amount of unknown solute and cyclohexane. We will
determine the molar mass of the unknown solute based on the decrease in the freezing point.

Introduction:
Several important properties of solutions depend on the number of solute particles in the
solution and not on the nature of the solute particles. These properties are called
COLLIGATIVE PROPERTIES, Colligative properties are the physical changes that result
from adding solute to a solvent. Colligative Properties depend on how many solute particles
are present as well as the solvent amount, but they do NOT depend on the type of solute
particles, although do depend on the type of solvent. The colligative properties are: vapor
pressure lowering, boiling point elevation, freezing point depression,
and osmotic pressure

Freezing point depression:

It results from the introduction of solute molecules to a solvent. When a pure solvent freezes,
its particles become more ordered as the intermolecular forces that operate between the
molecules become permanent. By dissolving a solute into the liquid solvent, this ordering
process is disrupted (entropy increases). As a result, more energy must be removed from the
solution in order to freeze it, and the freezing point of the solution is lower than that of the
pure solvent. The decrease in freezing point, Δ Tf, when a non-volatile, nonionizing (no
dissociating) solute is dissolved in a solvent is proportional to the molal concentration, m, of
the solute present in the solution.[1] .

The proportionality constant, Kf, is called the molal freezing-point depression constant. It
is a constant that is equal to the change in the freezing point for a 1-molal solution of a non-
volatile molecular solute. Every solvent has a unique molal freezing-point depression
constant. Molality is used in freezing depression points for colligative properties since the
colligative properties only depend on the number of particles in the solution. Molality, much
like molarity, does not depend on the temperature. The volume changes slightly if the
temperature changes, and the molarity will also change.

Boiling point elevation by the addition of a solute in the solvent the boiling point of the
solution elevates because the ions form an attraction with solvent particles that prevent the
water molecules from going into the gas phase. the amount of change in the boiling point is
related to the number of particles of solute in a solution and is not related to the chemical
composition of the solute. [2]
Figure 1 Naphthalene
Apparatus and chemicals:
• Freezing point apparatus
• Thermometer
• Ice
• Beakers (500ml)
• Water bath
• Cyclohexane
• Naphthalene (C10H8)
•
Procedure:
The apparatus was set up with ice.25 gram of cyclohexane was taken in a clean, dry freezing
tube of the apparatus and the freezing point of cyclohexane was found with constant stirring.
With every passing minute the reading on the thermometer was recorded and when the
reading because constant for successive concordant readings it was regarded as the freezing
point of cyclohexane.The freezing point was noted down. Since cyclohexane is in liquid form
so 32 ml was taken in view of 25 grams. The freezing tube was taken out and the solvent was
allowed to melt completely In order to make the process a little faster and to ease the process
it was kept in a water bath.
As soon as the solvent melted,0.1 gram of naphthalene was weighed and added to the solvent
and dissolved completely.
Again it was allowed to freeze and the above procedure was repeated of recording the
temperatures after every minute until the appearance of constancy. The above steps were
repeated for total 7 samples of naphthalene adding to the previous solution. The result was
tabulated and molar weight of naphthalene was calculated using the formula,

𝐾𝐾𝑓𝑓 𝑔𝑔 1000
𝑀𝑀 =
△ T𝑓𝑓W
Here,
g= weight of solute
w= weight of solvent
Kf= molar freezing depression constant(20 for cyclohexane)
M= molar weight of solute
Tf= depression in Freezing point
A graph was plotted of ∆Tf Vs g and slope was found and the results were compared
𝐾𝐾𝑓𝑓 𝑔𝑔 1000
△ 𝑇𝑇𝑓𝑓 =
MW
𝐾𝐾𝐾𝐾 1000
𝑚𝑚(𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠) =
𝑀𝑀𝑀𝑀
Observations and Calculations
△Tf ∝ m
…..(m = molality
△Tf = depression in freezing point)
△Tf ∝ Kf m ……(Kf = molal freezing point depression constant )
𝐾𝐾𝑓𝑓 𝑔𝑔 1000
△ 𝑇𝑇𝑓𝑓 =
MW
(g = wt. of solute
M = molar wt. of solute
W = wt. of solvent)
𝐾𝐾𝑓𝑓 𝑔𝑔 1000
𝑀𝑀 =
△ T𝑓𝑓W
Theoretical calculation
Molal freezing point depression constant Kf = 20°Ckg/mol
Weight of solvent = W = 25g
Freezing point of Cyclohexane = T1 = 6°C
Weight of Freezing point Depression of Freezing 𝐾𝐾𝑓𝑓 𝑔𝑔 1000
Naphthalene(g) of mixture (C) 𝑀𝑀 =
Point (△Tf ) △ T𝑓𝑓W
1. 0.1 5 1 80
2. 0.2 3 3 42.1
3. 0.3 1 5 48

80+42.1+48
Average molecular weight of Naphthalene = 3
= 56.7g/mol

Actual molecular weight of Naphthalene = 128.17 g/mol

56.7 −128.17
Percentage error = 128.17
× 100 = 55.76%

Graphical Calculations
𝐾𝐾𝑓𝑓 𝑔𝑔 1000
△ 𝑇𝑇𝑓𝑓 =
MW
Applying straight line equation y=mx+c
𝑦𝑦 = △ 𝑇𝑇𝑓𝑓
𝑥𝑥 = 𝑔𝑔
𝐾𝐾𝐾𝐾 1000
𝑚𝑚(𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠) = 𝑀𝑀𝑀𝑀
= 1.5714
c(intercept) = 0
1000 𝐾𝐾𝐾𝐾
𝑀𝑀 =
𝑊𝑊 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
1000 (20)
𝑀𝑀 = = 509.1g/mol
(25)(1.5714)

Discussion:
The reason for the depression of the freezing point is that according to Raoult’s Law, the
vapour pressure of a pure solvent reduces by dissolving a solute into the solution. The
freezing point of a substance is the temperature at which liquid and solid phases of a
substance co-exist in equilibrium. At this temperature, liquid and solid phases of a substance
have the same vapour pressure. Solute particles effect the freezing point of a pure solvent. A
pure liquid has only one type of particles. But when a non-volatile and non-electrolyte solute
is added to a solvent, its vapour pressure is decreased. This is because in solution both solute
and solvent particles occur on the surface. Thus solute particles decrease the number of
solvent surface particles. This decreases the vapour pressure, decrease in vapour pressure os a
pure solvent on the addition of a solute also affects the freezing point of the solution. The
solution will freeze at temperature at which vapour pressure of both solution and solid solvent
are the same. This means solution will freeze at a lower temperature than that of pure solvent.
Thus addition of a non-volatile solute also causes a decrease or depression in freezing point
of a solution.
We know that vapour pressure of a non-volatile solvent is zero. Therefore, the total vapour
pressure of the solution is lower than the vapour pressure of the pure solvent. The value of 𝐾𝐾𝑓𝑓
(molal freezing point constant or the cryoscopic constant) depends upon the nature of the
solvents and is independent of the solute used. The nature of solute does not affect the
freezing point depression as long as the solute doesn’t ionize.
The relationship between freezing point depression (∆𝑇𝑇𝑓𝑓 ) and weight of solute can be
depicted in a graph. This graph typically shows a linear relationship, indicating that as weight
of the solute increases, the freezing point depression also increases, This phenomenon is a
result of colligative properties of solutions, where the addition of non-volatile solute to a
solvent decreases the freezing point of the solvent. The graph helps visualize the extent to
which the solute affects the freezing point. Moreover, the difference between the theoretical
and experimental values of molar mass of solute can be due to various factors like impurities
in the solute, assumptions made during the theoretical calculation, or experimental errors in
measurement.
Conclusion:
The purpose of performing a depression in freezing point experiment is to determine the
molecular weight of solute in a solvent. This experimental technique is based on the principle
that adding a non-volatile solute to a solvent lowers the freezing point of the solution. By
measuring the extent of depression of freezing point, one can calculate the molecular mass of
solute, providing valuable information about its properties and composition.
References

[ [Online]. Available: https://flexbooks.ck12.org/cbook/ck-12-chemistry-flexbook-

1 2.0/section/16.14/primary/lesson/freezing-point-depression-chem/.
]
Date and Time: 3/11/23(4:05pm)

[ [Online]. Available:
2 https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry/13%3A
] _Solutions/13.09%3A_Freezing_Point_Depression_and_Boiling_Point_Elevation.

Date and Time : 29/10/23(10:00pm)