Freezing Point Depression: Molecular Weight Determination

Manahan, Sharmaine E.Group 4BS Chemistry III

College of Science and MathematicsWestern Mindanao State UniversityNormal Road, Baliwasan, Zamboanga City

ABSTRACT

In the experiment conducted, the freezing temperature of the mixture of glacial acetic

acid and naphthalene, unknown and glacial acetic acid solution, and pure glacial acetic acid

which is the organic solvent, were measured. It has been observed that when a solute was added

to the solvent, the freezing point of such a solution is lower than that of the pure solvent.

Based on the results gathered, the molecular weight of naphthalene was 125.8 g/mol +

0.41 and for unknown, 61.63 g/mol + 24.4. Thus, the theoretical molar mass of naphthalene is

128.2 g/mol. Since the standard deviation of the molar mass of naphthalene was + 0.41, it means

that there is a small spread of data due to its calculated value which is less than 1. On the other

hand, the molecular weight of the unknown has a standard deviation + 24.4, it implies that the

data obtained was not precise and there is a large spread of data.

The molar mass of the substance was determined, and the freezing point of that solution

to that of pure solvent was able to compare. The data collected was graphed to show how the

solutes affect the freezing point of a substance.

KEYWORDS: Freezing point, Molecular Weight, Temperature, Standard Deviation

INTRODUCTION

Solutions are homogeneous mixtures that contain two or more substances. The major

component is called the solvent, and the minor component is called the solute. Since the solution

is primarily composed of solvent, physical properties of a solution resemble those of the solvent.

Some of these physical properties, called colligative properties, are independent of the nature of

the solute and depend only upon the solute concentration, measured in molality, or moles of

solute per kilogram of solvent. The colligative properties include vapor pressure lowering,

boiling point elevation, freezing point depression, and osmotic pressure. The vapor pressure is

the escaping tendency of solvent molecules. When the vapor pressure of a solvent is equal to

atmospheric pressure, the solvent boils. At this temperature, the gaseous and liquid states of the

solvent are in dynamic equilibrium, and the rate of molecules going from the liquid to the

gaseous state is equal to the rate of molecules going from the gaseous state to the liquid state.

The new freezing point of a solution can be determined using the colligative property

law:

ΔTf = kf m

The change in freezing point is equal to the molal freezing-point constant times the molality of

the solution. The molal freezing-point constant used is the constant for the solvent, not the solute.

In this experiment, the molar mass of sulfur will be determined using the colligative

property law. The freezing point of naphthalene will be determined experimentally; then a

controlled solution of naphthalene and unknown will be made, and the freezing point of that

solution will be determined. The difference in freezing point can be used in the colligative

property law to determine the experimental molality of the solution, leading to a calculation of

molecular weight.

The freezing temperature is difficult to ascertain by direct visual observation because of a

phenomenon called supercooling and also because solidification of solutions usually occurs over

a broad temperature range. Temperature-time graphs, called cooling curves, reveal freezing

temperatures rather clearly.

METHODOLOGY

In this experiment, 5-mL glacial acetic acid, naphthalene, unknown, dewar flask, becky

thermometer, stopwatch, pipette, aspirator, salt for the ice bath, outer and inner freezing point

tubes, and 100-mL beakers were used.

The small dewar flask was filled with ¾ of ice-water mixture. Five mL of acetic acid was

placed into the inner freezing point tube. After doing so, the inner freezing point tube was placed

inside the outer freezing point tube and put in an ice bath. The temperature reading from the

thermometer was recorded every 30 seconds. The solution was stirred up and down to ensure

even freezing. When the temperature remained constant for several readings, the solution was

allowed to cool without further temperature readings. The inner tube warmed with hand until all

the crystals have melted. After this, 0.10-mL of glacial acetic acid was added and the same

technique was applied. Then, 0.05-mL of glacial acetic acid has been added to the acetic acid and

the same procedure was repeated until 0.30-mL of glacial acetic was already added. For

naphthalene and unknown, these solutes were added separately after allowing the solvent to

warm with hands.

RESULTS AND DISCUSSION

Table 1.1 shows the experimental and theoretical weight of different substances with

respect to its constant temperature.

Solution Weight/mL Constant temperature

Molecular weight% ErrorExperimental Theoretical

(naphthalene)

Pure Glacial Acetic acid

5.00 mL + 0.01

14.2oC + 0.01

125.8 g/mol + 0.41

128.2 g/mol 1.87%Naphthalene 0.5080 g -

Pure Glacial Acetic Acid + Naphthalene

- 11.2oC + 0.01

For unknown

Pure Glacial Acetic Acid

5.00 mL + 0.01

14.32oC + 0.01

61.63 g/mol + 24.4

- -Unknown 0.5112 g -

Pure Glacial Acetic acid +

Unknown- 7.52oC + 0.01

The known molar mass of naphthalene is 128.2 g/mol. The calculated molar mass of

naphthalene was 125.8 g/mol + 0.41. A percent error calculation can help to measure the

accuracy of the experiment and it has been found out that the percent error was 1.87%. It

indicates that the experimental molecular weight of naphthalene is accurate due to its small

percentage of error. This error is attributable to several sources of error that were present in this

experiment. While the imprecision of instruments is not technically a source of error, it had a

particularly effect on this experiment. Additionally, the transfer of powdered naphthalene from

the weighing paper to the test tube may have been incomplete. Some particles of naphthalene

may have been lost or may have gone unreacted due to the imperfection of the transfer method.

The standard deviation value which is + 0.41 implies that the experiment conducted was precise

and there is a small spread of data.

The molecular weight of the unknown solution was also determined experimentally. But

the standard deviation gave a value of + 24.4 which means that a large spreading of data was

done. To determine the molar mass of a substance, one must simply divide the grams of

substance by the number of moles of substance present. All of these values were determined

experimentally.

In the figure 1.1, the variation of temperature of pure glacial acetic acid, and solution of

naphthalene with glacial acetic acid was shown graphically with respect to the time interval.

0 100 200 300 400 500 6000

5

10

15

20

25

30

35

f(x) = − 0.0355392156862745 x + 23.6176470588235R² = 0.639161363544242f(x) = − 0.0307692307692308 x + 22.1153846153846R² = 0.616796695731987

Temperature reading vs. TimeTemperature reading (pure Acetic acid)

Linear (Temperature reading (pure Acetic acid))

Temperature reading (Acetic acid with Naphthalene)

Linear (Temperature reading (Acetic acid with Naphthalene))

Time, sec

Tem

pera

ture

Figure 1.1 Reading of Temperature (pure Acetic acid, Acetic acid with naphthalene) vs. Time

The theory associated with this experiment is the atomic theory of matter. The atomic

theory of matter offers explanations for bonding and physical phase. The figure 1.1 presented

above illustrates the difference between pure substance and a solution. The freezing point of a

solution of glacial acetic acid with naphthalene is lower than the freezing point of the pure

substance. Because when a solute is present, the surface of the solution is comprised of solute

particles and solvent particles, instead of only solvent particles. When the vapor pressure of a

solution is lowered, the freezing point is lowered. Because the number of molecules of solute has

a direct effect on the rate of evaporation, the freezing point depression of a solution is

proportional to the molality of the solution.

The same case happened in the unknown substance. When unknown was added into the

pure acetic acid the freezing point decreases than that of pure solvent. To visualize the effect of

the addition of unknown to the solvent, a graph was encapsulated in figure 1.2.

In figure 1.2, the variation of temperature of pure glacial acetic acid, and solution of

unknown with glacial acetic acid was presented graphically with respect to the time interval.

0 100 200 300 400 500 6000

5

10

15

20

25

30

35

f(x) = − 0.040034965034965 x + 20.0641025641026R² = 0.631349075588061

f(x) = − 0.0452380952380952 x + 26.7582417582418R² = 0.817442452501541

Temperature reading vs. TimeTemperature Reading (pure Acetic acid)

Linear (Temperature Reading (pure Acetic acid))

Temperature Reading (Acetic acid with unknown)

Linear (Temperature Reading (Acetic acid with unknown))

Time, sec

Tem

pera

ture

Figure 1.2 Reading of Temperature (pure Acetic acid, Acetic acid with unknown) vs. Time

There are many ramifications associated with this experiment. One is the personal

experience using colligative equations was gained. Also techniques to prevent supercooling were

practiced.

CONCLUSION

In this experiment, the freezing temperature of the pure solvent, glacial acetic acid was

found first. Next, the freezing point of a solution prepared from a mixture of an unknown organic

compound and glacial acetic acid was measured. Also, the freezing point of naphthalene and

acetic acid mixture was assessed too. It is true that the vapor pressure of volatile solvents is

lowered when a non-volatile solute is used to make a solution. The result is that such a solution

has a lower freezing point than that of the pure solvent.

Based on the results obtained, the molecular weight of naphthalene was 125.8 g/mol +

0.41 and for unknown, 61.63 g/mol + 24.4. For comparison, the actual molar mass of

naphthalene is 128.2 g/mol. Since the standard deviation of the molar mass of naphthalene was +

0.41, it implies that there is a small spread of data because it is less than 1. On the other hand, the

molecular weight of the unknown has a standard deviation + 24.4, it implies that the data attained

was not precise and there is a very large spread of data.

The determination of the molar mass of the substance, and the comparison of the freezing

point of that solution to that of pure solvent were achieved. The experiment was not very

accurate, but it was still successful in terms of the original purpose.