freezing point depression method

GCSE Maths

Experiment 5

Title:

Determining molecular weight by freezing point depression method.

Objectives:

- To determine freezing point of a substance from cooling curve.

Study effect of foreign substance content on freezing point of a solvent.
Determine molecular weight using freezing point depression.

Theory & Background:

Dissolution of a substance is a solvent suppresses the freezing point of the solution formed. The freezing depression depends on the amount of the material dissolved in the solvent according to the following equation:-

∆T = Kf m = Kf x ____ __mass of material_______

Molecular weight X Kg solvent

Where,

∆T = freezing point depression

Kf = freezing point molar constant of solvent

m = molal concentration = mass of material/molecular weight x kg solvent

= No.of moles of solute/kg solvent

Naphthalene is a crystalline, aromatic, white, solid hydrocarbon, best known as the primary ingredient of mothballs. Naphthalene is volatile, forming a flammable vapor. Its molecules consist of two fused benzene rings. It is manufactured from coal tar, and converted to phthalic anhydride for the manufacture of plastics, dyes and solvents. It is also used as an antiseptic and insecticide, especially in mothballs. p-Dichlorobenzene is now often used instead of naphthalene as a mothball substitute. Naphthalene easily sublimates at room temperature. The structure of naphthalene is shown on the next page:

Apparatus and Materials:

Naphthalene
Substance X
Test tubes
Beaker (500cm3)
Thermometer
Glass rod

Procedures:

Part 1 : Determination of the freezing point of naphthalene

1) A clean and dry test tube was weighed.

2) About 5 g of naphthalene was added to the test tube and weighed again.

3) The naphthalene was melted by using a water bath.

4) When all has melted, the test tube was transferred from the water bath, dried and clamped on a retort stand.

5) The liquid was stirred continuously so that no temperature gradient develops and the temperature was recorded every 15 seconds for 5 minutes and then every 30 seconds for another 10 - 15 minutes.

6) A graph of temperature versus time was plotted.

7) The freezing point of naphthalene was determined.

8) Melting and cooling steps were repeated for the same sample. (Steps 3 - 7)

Part 2 : Determination of the freezing point and molecule weight of a substance

1) Substance X provided was weighed accurately 0.5 - 0.75 g.

2) This substance was added to naphthalene in a test tube which was used in (1).

3) Melting and cooling steps were carried out. (Steps 3 - 7 above were repeated).

4) The melting and cooling were repeated by using different masses of substance X

(i.e. by adding 0.15 - 0.25 g of X which is accurately weighed) to the naphthalene -X mixture which had been used above.

5) The molecular weight of substance X was calculated as an average of two determinations. [Kf for naphthalene is 6.8]

Data & Result:

Mass of naphthalene: 5.00g

Mass of substance X (0.50 to 0.75g): 0.50g

Mass of substance X (0.50 to 0.75g + 0.15g to 0.25g): 0.65g

Freezing point of naphthalene: 78 oC

Freezing point of substance X (0.50 to 0.75g): 77oC

Freezing point of substance X (0.50 to 0.75g + 0.15g to 0.25g): 76oC

Table 1: Freezing point/melting point of naphthalene, naphthalene + (0.50 to 0.75g X) and naphthalene + (0.50 to 0.75g X) + (0.15 to 0.25g X) from 0 - 150 seconds

Table 2: Freezing point/melting point of naphthalene, naphthalene + (0.50 to 0.75g X) and naphthalene + (0.50 to 0.75g X) + (0.15 to 0.25g X) from 180 - 480 seconds

Table 3: Freezing point/melting point of naphthalene, naphthalene + (0.50 to 0.75g X) and naphthalene + (0.50 to 0.75g X) + (0.15 to ...