POSTLAB Experiment No.3

ChE321L Experiment No. 3

ChE321L Experiment No. 3

DETERMINATION OF THE

DETERMINATION OF THE

MOLECULAR WEIGHT OF A

MOLECULAR WEIGHT OF A

NON-VOLATILE SOLID BY 

NON-VOLATILE SOLID BY 

THE CRYOSCOPIC METHOD

THE CRYOSCOPIC METHOD

Prepared by:

Prepared by:

GROUP No. 3

GROUP No. 3

LUNK 

LUNK , Michael Angelo, Michael Angelo

 YUSON

 YUSON, Joana Marie N., Joana Marie N.

University of Santo Tomas

University of Santo Tomas

Faculty of Engineering

Faculty of Engineering

Chemical Engineering Department

I. INTRODUCTION

I. INTRODUCTION

 The objective of this experiment

 The objective of this experiment

is to determine the molecular

is to determine the molecular

weight of an unknown solute using

weight of an unknown solute using

the cryoscopic method or freezing

the cryoscopic method or freezing

point depression method.

I. INTRODUCTION

I. INTRODUCTION

 The objective of this experiment

 The objective of this experiment

is to determine the molecular

is to determine the molecular

weight of an unknown solute using

weight of an unknown solute using

the cryoscopic method or freezing

the cryoscopic method or freezing

point depression method.

The general definition of freezing

The general definition of freezing

point depression is

point depression is

the effect of 

the effect of 

lowering the freezing point of a

lowering the freezing point of a

substance due to an increased

substance due to an increased

amount of solute added to the

amount of solute added to the

solvent

solvent

.

.

This

This

principle

principle

can

can

be

be

explained in three primary equations.

 These are:

 These are:

ΔT

ΔT

=

=

T

T

- T

- T

K 

K 

=

=

ΔT

ΔT

MW

MW

m

m

//

m

m

MW

MW

=

=

K 

K 

m

m

/ΔT

/ΔT

m

m

Equation 2 can also be rearranged to Equation 3 Equation 2 can also be rearranged to Equation 3

Equation 3

  The Cryoscopic constant,

K 

(of Glacial

acetic

acid)

was

determined.

We

first

determined the freezing point of the pure

glacial acetic acid, and then the freezing point

of the solutions containing measured masses

of glacial acetic acid and benzoic acid. From

these experimental data, we have calculated

the

K 

of Glacial Acetic Acid.

We have prepared a solution of 

known

masses

of

an

unknown

substance dissolved in the Glacial

acetic acid and and determined the

freezing point of the solution. From

these data, we have calculated the

molar mass (MW) of the unknown

substance.

After the experiment, it

was realized that the H+ ions

within the solutes used have

played a big role in the

freezing point depression of 

the solution.

2. TRANSFERRED in the hard glass test tube

II. PROCEDURE

A. Determination of Cryoscopic Constant of the Solvent

15mL Glacial Acetic Acid

Crushed Ice

All contents SOLIDIFIED

All contents LIQUEFIED

1. PLACED in the beaker

5. TRANSFERRED into an empty beaker

6. RECORDED temperature reading (15 seconds interval)

3. DIPPED in the ice-water mixture

4. RECORDED temperature reading

2. TRANSFERRED in the hard glass test tube

15mL Glacial Acetic Acid

Crushed Ice

1. PLACED in the beaker

3. ADDED with 1-2 grams Benzoic Acid 4. STIRRED Benzoic Acid is Completel y

Dissolved5. DIPPED in the ice-water_mixture

6. RECORDED temperature reading

(15 seconds interval)

All contents SOLIDIFIED

All contents LIQUEFIED

7. TRANSFERRED into an empty beaker 8. RECORDED temperature reading

(15 seconds interval)

*This procedure was done twice using varied masses of Benzoic Acid (between 1.0g -1.5g) and a new volume of Glacial Acetic Acid

5. DIPPED in the ice-water mixture 6. RECORDED temperature readin

(15 seconds interval)

B. Determination of the Molecular Weight of the Unknown Solute

2. TRANSFER in the hard glass test tube

15mL Glacial Acetic Acid

Crushed Ice

1. PLACED in the beaker

3. ADDED with 1-2 grams unknown solute

4. STIRRED

UNKNO WN SOLUTE

All contents SOLIDIFIED

All contents LIQUEFIED

7. TRANSFERRED into an empty beaker 8. RECORDED temperature reading

(15 seconds interval)

*This procedure was done twice using varied masses of unknown

solute (between 1.0g

-1.5g) and a new volume of Glacial Acetic Acid.

III. DATA AND

RESULTS

 Table 1. Temperature readings to determine the freezing point he pure Acetic Acid (T_solvent ) and Acetic-Benzoic Acid solutions (T_solution

Mixture

t/mm:s

s

T/°C

Observation

15mL Pure

Glacial Acetic

Acid

01:30

15

14:45

15.5

Pure solid

00:45

17

Pure liquid

02:00

10

18:30

12.5

Pure solid

42:30

17

Pure liquid

A. DETERMINATION OF THE CRYOSCOPIC CONSTANT OF THE SOLVENT

Figure 1. Cooling curve for pure Glacial

Acetic Acid.

Figure 2. Cooling curve for the solution of 

Benzoic Acid in Glacial Acetic Acid.

 Time (minutes) T e m p e ra tu re (° C ) Pure liquid Pure solid Liquid - solid First crystals appears

Table 2. Temperature readings to determine the freezing poi of the Glacial Acetic Acid- Unknown substance solutions (T_solution

Mixture

t/mm:s

s

T/°C

Observation

02:30

12.5

04:15

13

Pure solid

37:15

18

Pure liquid

01:15

11.5

12:30

11.5

Pure solid

B. DETERMINATION OF THE MOLECULAR WEIGHT OF THE UNKNOWN SOLUTE

Figure 3. Cooling curve for the solution of 

Unknown solute in Glacial Acetic Acid.

Pure liquid

Pure solid

Liquid - solid First crystals appears

COMPUTATIONS

*The following formulae were used to obtain the

required values

in Procedure A:

Where:



ΔT

is the lowering of the freezing point in

°C.



T

puresolvent

is the freezing point of pure

solvent in °C

ΔT

=

T

- T

m=

ρV

m

is the mass in g.

ρ

is the density in

g/mL.

V

is the volume in

mL.

Where:

Where:



K 

is the cryoscopic constant in °C kg/

mole.



ΔT

is the lowering of the freezing point in

°C.



MW

is the molecular weight of solute

in °C.



m

is the mass of solvent in kg.

m

is the mass of solute in g.

K 

=

ΔT

MW

m

/

m

*The following formulae were used to obtain the

required values

in Procedure B:

m=

ρV

ΔT

=

T

- T

MW

=

K 

m

/ΔT

A. DETERMINATION OF THE CRYOSCOPIC CONSTANT (K _f ) OF THE GLACIAL ACETIC ACID

•

MW

• T_puresolvent : T_{CH 3COOH} = 15.5

°C

•

ρ

ρ

(from Atkin’s Data Section on page 990)

TRIAL 1

• V_{CH 3COOH} = 15 mL;

since

m= ρV

,

• m

= 0.015735 kg

• m

= 1.4100 g

• T

= 12.5

°C

ΔT

=

T

 – T

=

°C -

°C

f 

=

ΔT

MW

m

m

3

°C (122 g

/mole

) (0.015735 kg

1.4100 g

•

ΔT

=

°C

•

MW

=

m

=

m

=

A. DETERMINATION OF THE CRYOSCOPIC CONSTANT (K _f ) OF THE GLACIAL ACETIC ACID

•

MW

• T_puresolvent : T_{CH 3COOH} = 15.5

°C

•

ρ

ρ

(from Atkin’s Data Section on page 990)

TRIAL 2

• V_{CH 3COOH} = 10 mL;

since

m= ρV

,

• m

= 0.01049 kg

• m

= 1.4916 g

• T

= 11

°C

ΔT

=

T

 – T

=

°C -

°C

f 

=

ΔT

MW

m

m

4.5

°C (122 g

/mole

) (0.01049 kg

1.4916 g

•

ΔT

=

°C

•

MW

=

m

=

m

=

K 

= 3.86 °C kg/mole

K 

= 4 °C

kg/mole

K 

= 3.93 °C kg/mole

+

2

Average

K 

:

=

=

B. DETERMINATION OF THE MOLECULAR WEIGHT OF THE UNKNOWN SOLUTE

•

K 

°C kg/mole

• T_solvent : T_{CH 3COOH} = 15.5

°C

•

ρ

ρ

(from Atkin’s Data Section on page 990)

TRIAL 1

• V_{CH 3COOH} = 15 mL;

since

m= ρV

,

• m

= 0.015735 kg

• m

= 1.2768 g

• T

= 13

°C

ΔT

=

T

 – T

=

°C –

MW

=

K 

m

ΔT

m

=

°C kg

/mole

(1.2768 g

)

2.5

°C (0.015735 kg

)

•

ΔT

=

°C

•

K 

=

°C kg/mole

•

m

=

m

=

B. DETERMINATION OF THE MOLECULAR WEIGHT OF THE UNKNOWN SOLUTE

•

K 

°C kg/mole

• T_solvent : T_{CH 3COOH} = 15.5

°C

•

ρ

ρ

(from Atkin’s Data Section on page 990)

TRIAL 2

• V_{CH 3COOH} = 10 mL;

since

m= ρV

,

• m

= 0.01049 kg

• m

= 1.3745 g

• T

= 11.5

°C

ΔT

=

T

 – T

=

°C –

MW

=

K 

m

ΔT

m

=

°C kg

/mole

(1.3745 g

)

4

°C (0.01049 kg

)

•

ΔT

=

°C

•

K 

=

°C kg/mole

•

m

=

m

=

MW

= 128.74 g/mol

MW

= 127.56

g/mole

MW

= 128.15 g/mole

+

2

Average

Molecular Weight :

=

Average

Molecular Weight :

Average

K 

:

PERCENT ERROR = | Experimental – Theoretical | Theoretical % error = |3.93

°C kg/mole

°C

kg/mole|

°C kg/mole

%

% error = |128.15

g/mole

g/mole|

128.1632

g/mole

% error = 0.01 %

V. ANSWERS TO

QUESTIONS

1.

From

the

plot

of 

temperature vs. time, the

freezing

point

can

be

determined by observing the

lowest point on the curve. It is

the indication when freezing is

nearly to occur.

2. Based on the results, the

freezing point of pure acetic acid

is relatively higher compared to

the solutions’ freezing point after

a certain amount of solute is

dissolved in the acetic acid. As

the solvent crystallizes, the solute

concentration increases, resulting

in further lowering of freezing

temperature.

3. The calculated molecular

weight of the unknown solute

was not affected by the amount

of acetic acid used. Instead, it is

the freezing point that was

affected by the amount of the

acetic acid. This can be proven

by the equation:

ΔT

=

K 

m

MW

m

So if the amount of glacial

acetic acid used was more

than 15mL, then the freezing

temperature

would

have

become higher and lower if it

was less than 15mL.

4. If one is to guess what the

solid sample is without looking

at the result of the experiment,

the first logical clue that must

be considered is its smell since

all the solidified solutions in

this experiments looks the

same.

5.

Supercooling

is

a

phenomenon where in a liquid

cools below its freezing point

before crystallization occurs.

  This phenomenon has actually

occurred in all parts of this

experiment as explained in the

graphs.

In the addition of the

naphthalene to the Glacial acetic

acid solution. The H

_{ions within}

the

naphthalene

cause

the

freezing point to lower because

the ions act to disrupt the bonds

between the particles.

VI. CONCLUSION AND

RECOMMENDATION

The percent error was calculated

to be relatively low (around 0.01%).

Errors that contributed to this could

include impurities in the Glacial

acetic acid-naphthalene mixture and

imprecise readings of temperature

and masses of substances.

REFERENCES

*Atkin’s Physical Chemistry 8

_Edition

*Physical Principles 2 Laboratory Manual

*Leider’s Physical Chemistry 3

_Edition

*http://www.ncbi.nlm.nih.gov/pmc/articles/PMC125

3340/pdf/biochemj01101-0190.pdf 

*http://www.chemistry.ccsu.edu/glagovich/teachin

g/31

Thank you for

listening!