Chem 17 Full Report

7/27/2019 Chem 17 Full Report

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Marie Antoinette Torres Nov. 27, 2012

A 20L Dec. 4, 2012

Group 3

Colligative Properties

I. Introduction

A phase change is the change from one physical state to another, accompanied by a change in

energy of the system. From an ordered state, energy must be supplied to overcome the intermolecular

forces of attraction to transform the substance into a less ordered state. The phase changes involved in

this exercise were melting, freezing, and boiling.

Melting is the conversion of a substance from solid to liquid. An increase in temperature and

kinetic energy disrupts the arrangement of the solid until the intermolecular forces of attraction are

overcome, resulting in the fluid motion of the particles. Freezing is the conversion of a substance from

liquid to solid. Due to the decrease in temperature and kinetic energy of the liquid, a structure is formed

and the liquid substance becomes solid. Boiling is the change from liquid to gas. It occurs when the

vapor pressure of the liquid becomes equal to the atmospheric pressure.

Colligative properties are the properties of solutions that are dependent not on the nature of

the solute, but on the amount present. The properties involved in this exercise were boiling point

elevation and freezing point depression. In boiling point elevation, the boiling point of the solution

increases as the concentration increases. In freezing point depression, the presence of a solute

decreases the freezing point of the solution.

The objectives of this exercise were:

1. to describe colligative properties of solutions;

2. to determine the effect of solute concentration on boiling point and freezing point of

the solution; and


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3. to compute for the molar mass of an unknown solute using freezing point depression

data.

II. Materials

A. Reagents

Naphthalene

B. Apparatus

Test tubes Iron stand

Iron clamp Iron Ring

Thermometer Bunsen burner

4 250-mL beakers

III. Procedure

Two grams of naphthalene were obtained and put in a test tube. Then, the tube was put in a

250 mL beaker. Distilled water was poured to the beaker until the water level is above the sample in the

test tube. The beaker was then heated until the sample has melted and the temperature reached 90:C.

After the sample has reached 90:C, the beaker was put off the flame. Its temperature was then

recorded at 15 second intervals until the temperature reached 70:C.

0.20 g of an unknown substance and 2.0 g of naphthalene were mixed in a test tube. The tube

was then put in a 250 mL beaker with a water level just above the sample. The beaker was then heated

until the sample has melted and the temperature reached 90:C. After reaching that temperature, the

beaker was put off the flame. The temperature of the sample was recorded every fifteen seconds until it

dropped to 70:C.


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100 mL of distilled water was obtained and put into a 250 mL beaker. The distilled water was

then heated and its temperature recorded at 15 second intervals until it has reached its boiling point.

100 mL of distilled water was obtained and transferred into a 250 mL beaker. 1.5 g of an

unknown sample was obtained and dissolved into the water. The solution was then heated. Itstemperature was recorded every fifteen seconds until the solution has reached its boiling point.

IV. Data and Observations

Table 1.1. Determination of the Freezing Point of Naphthalene

Time (s) Temperature (:C) Phase Time Temperature (:C) Phase

0 90 Liquid 315 78 Liquid

15 90 Liquid 330 77 Liquid-Solid




75 87 Liquid 390 76 Solid














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Figure 1. Cooling Curve of Naphthalene

Table 1.2. Determination of the Freezing Point Depression of Naphthalene










0

10

20

30

40

50

60

70

80

90

100

030

60

90

120

150

180

210

240

270

300

330

360

390

420

450

480

510

540

570

600

Naphthalene

Naphthalene


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150 82 Liquid

165 81 Liquid

180 81 Liquid

195 80 Liquid

210 80 Liquid

225 79 Liquid

240 78 Liquid

255 77 Liquid

270 77 Liquid

285 76 Liquid

300 76 Liquid-Solid

315 75 Liquid-Solid

330 75 Liquid-Solid

345 75 Liquid-Solid

360 75 Liquid-Solid

375 74 Solid


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Figure 2. Freezing Point Depression of Naphthalene

Table 1.3. Determination of the Boiling Point of Distilled Water






60 47 Liquid 225 93 Liquid-Gas


90 58 Liquid 255 98 Gas

105 60 Liquid 270 98 Gas

120 64 Liquid 285 98 Gas

135 68 Liquid

150 72 Liquid

0

10

20

30

40

50

60

70

80

90

100

030

60

90

120

150

180

210

240

270

300

330

360

390

420

450

480

510

Napthalene + Unknown

Napthalene + Unknown


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Figure 3. Heating Curve of Distilled Water

Table 1.4. Determination of the Boiling Point of Distilled Water + Unknown solute

Time Temperature Phase Time Temperature Phase







90 56 Liquid 270 99 Gas

105 60 Liquid 285 99 Gas

120 65 Liquid 300 99 Gas

135 69 Liquid 315 99 Gas

150 74 Liquid 330 99 Gas

165 78 Liquid

0

20

40

60

80

100

120

015

30

45

60

75

90

105

120

135

150

165

180

195

210

225

240

255

270

285

300

Distilled water

Distilled water


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Figure 4. Heating Curve of Distilled Water + Unknown Solute

Table 1.5. The Boiling Point of distilled water and dH2O + unknown solute at different concentrations

Group Amount of Solute (g) Boiling Point of dH2O Boiling Point of dH2O +

unknown solute

1 0.5 99 100

2 1.0 98 98

3 1.5 98 99

4 2.0 98 99

5 2.5 96 97

0

20

40

60

80

100

120

0 30 60 90 120 150 180 210 240 270 300 330

distilled water + unknown solute

distilled water + unknown

solute


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V. Discussion

In the experiment, the freezing point of pure naphthalene was 76:C. The experimental value

was far from the actual freezing point of naphthalene. A possible source of error might be an incorrect

amount of sample used in the experiment.

The freezing point of the naphthalene solution acquired is 74:C. There was a 2:C difference

between the pure naphthalene and the naphthalene solution. The presence of a solute decreased the

freezing point of the solution. The presence of more solute makes it easier for the solution to crystallize

as it cools.

The molality of the solution is 0.29 m. This could be computed dividing the freezing point

depression by the molal freezing point depression constant.

The moles of solute used in the solution could then be computed.

The molar mass of the solute is 344.82 g/mol.

The boiling point of distilled water in the experiment is 98:C while the boiling point of the

solution is 99:C. The increase in the boiling point was caused by the presence of solute. The more

concentrated the solution becomes, the more crystalline is its structure. Because of the structure

becoming more ordered, it would take more energy to break the bonds of this structure. More energy

would result to an increase in boiling point.


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VI. Conclusion

The freezing point of a solution decreases as the amount of solute increases. More solute mixed

into a solution would create more bonds with the solvent molecules. As more bonds are formed, the

structure becomes more crystalline. If the structure is becoming more crystalline, then it would be

easier for the solution to take shape and freeze.

Boiling point is directly proportional to the amount of solute present in the solution. The

presence of solute increases the boiling point of the solution. As a solution becomes more concentrated,

more bonds are formed. An increased amount of energy is required to break these bonds. A higher

temperature is needed to produce an increased amount of energy.

VII. References

Guch, I. (2009). Fun With Colligative Properties. Available:

http://misterguch.brinkster.net/colligativepropertiestutorial.html

Helmenstine, A.M. Freezing Point Depression, What Freezing Point Depression Is And How It

Works. Available:

http://chemistry.about.com/od/solutionsmixtures/a/freezingpointde.-Nxc.htm

Laboratory Instruction Manual for Chem 16.1 General Chemistry I Laboratory. Los Banos,

Laguna: Institute of Chemistry, University of the Philippines Los Banos.
http://chemistry.about.com/od/solutionsmixtures/a/freezingpointde.-Nxc.htmhttp://chemistry.about.com/od/solutionsmixtures/a/freezingpointde.-Nxc.htmhttp://chemistry.about.com/od/solutionsmixtures/a/freezingpointde.-Nxc.htm

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