Determine the molecular weight of Naphthalene by RAST's method using camphor as a solvent.
Theory:
Mixing the non-volatile solute with the solvent lowers the freezing point of the solution. This is known as freezing point depression. To determine the molecular weight of a non-volatile solute, first, determine the freezing point (or melting point if the solvent is solid) of a pure solvent. Then dissolve a known weight (eg, the solute of w (naphthalene) in a solvent (camphor) of the known weight (eg, W). Heat and dissolve in a sealed tube to provide a homogeneous solution of the solids. Cool the tube and determine the melting point of the mixture, which is lower than the solvent. The difference between the two melting points gives depression at the freezing point. Knowing the value of the molar depression constant of camphor the molecular weight of naphthalene can be determined by this relationship.
Requirement: (i) Apparatus :
A small Pyrex glass test tube, test tube, stainless steel spatula, capillary tube, oil bath, balance, weighing box, melting point apparatus, and a cork with a hole to hold the thermometer.
(ii) Chemicals
Liquid paraffin (for oil path), camphor, Naphthalene.
Procedure:
Step I Preparation of a homogeneous mixture of Camphor and Naphthalene.
- Take a small Pyrex tube and clean and dry it. Put it in the hole of the cork so that it can stand upright in the pan of the balance. Note its weight accurately.
- Put about 20.0 mg of Naphthalene in the tube with help of a spatula. Note the weight of the tube again.
- Add about 200 mg of camphor to the tube and note its weight.
- Close the tube loosely with a rubber cork and place it in 100ml. The beaker contains liquid paraffin. Heat the beaker to approximately 150–180 °C to melt the contents of the tube. Stir it gently to get a homogeneous solution.
- Cool the tube in a water bath to avoid condensation of camphor in the upper parts of the test tube. Stir alternately until it solidifies when it cools.
- Open the tube and transfer the solid mixture to a clean watch glass or porous plate with the help of a nickel spatula. Grind well with a spatula.
Step II
Determination of melting points:
Determination of melting point of camphor (solvent).
- Take a small amount of pure dry camphor on a porous plate and powder with the help of a spatula.
- Take a fine capillary tube 5-6 cm long and close one end by heating it over a fire. Hold it horizontally and grate the camphor crust from the plate into the capillaries.
- Transfer the solid material to the sealed end by tapping the stands 3-4.
- Repeat the scraping and tapping process until the camphor is packed in the capillary stands 3-4 mm from the sealed end.
- Attach the capillary to the thermometer in the melting point path. Heat slowly and determine the melting point.
Observations:
Weight of empty pyrex tube = a grams
Weight of tube + Naphthalene = b grams
Weight of tube + Naphthalene + Camphor = c grams
Melting Point :
S.No. Melting point of mixture Melting point of camphor |
1. …...... …...... |
2. …....... …...... |
3. …....... …...... |
Mean T1 Mean T0 |
The melting point of pure camphor (average) = T0 ℃
Melting point of the mixture (average) = T1 ℃
Depression at freezing point = T0 - T1 ℃
= ∆Tf
General Calculations :
Weight of Naphthalene = b- a = w grams
Weight of Camphor = c- b = W grams
The molecular weight of the substance, m =
Molal depression constant K, for camphor = 39.7°C (given)
Molecular weight of the substance =
Precautions :
- While preparing homogeneous mixture of Naphthalene with camphor shake the contents of the ignition tube vigorously while heating so that a homogeneous solution is formed.
- The melting point is noted on the thermometer when the last trace of crystalline matter just disappears.
- Heating should be done very slowly.
- Avoid evaporation of camphor.
- Weighing should be done carefully.
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