How To Determine the percentage composition of unknown liquid by surface tension method?

 

Experiment:

Determine the percentage composition of unknown liquid by surface tension method.


 

Apparatus:

Stalagmometer, specific gravity bottle, screw type pinch cock, beakers, rubber tube, balance, stand etc.

Procedure:

The composition of this mixture can be determined as follows:

 

·         Prepare various compositions of the two liquids (water and a miscible organic liquid)

 

Sample No.                 1          2          3          4          5          6

Org. liquid (%)            100      80        60        40        20        0

Water (%)                    0          20        40        60        80        100

 

·         Find out the no. of drops of each composition using a stalagmometer (Drop Number Method).

·         Count the no. of drops of the given mixture in the same way as for the mixtures of known compositions.

·         Plot a graph between the %age compositions and their respective no. of drops.

·         By inserting the no. of drops for the given mixture, find the %age composition of given mixture of liquids.

 

 

 

 

 

 

 

 

 

 

 

Material Related to Multiple Choice Questions / Viva Voce

 

·         Surface tension of a liquid is also called Interfacial Tension.

·         The ratio of surface tension of a liquid to that of some standard liquid such as water is called Relative Surface Tension.

·         Stalagmometer is also called Drop Pipette.

·         Surface tension makes the surface as small as possible for a given volume of a liquid. Therefore drops and bubbles of liquid acquire a spherical shape (Among various geometric shapes, sphere has the minimum surface to volume ratio)

·         A drop falls from the lower end of a drop pipette when the weight of the liquid forming the drop becomes slightly greater than the total supporting surface tension (equal to 2p rg).

·         Angle between a drop of liquid and the surface of a solid is called Contact Angle. The angle between the surface of a liquid in the capillary tube and the wall of the tube is also the contact angle.

·         Volume occupied by the mole (molecular mass expressed in grams) of a liquid at a given temperature is called Molar Volume of the liquid.

·         Relative density is also called Specific Gravity which is the ratio between the density of a substance and the density of a standard substance (water).

·         Specific gravity of a substance = Density of substance/ Density of water

·         Parachor is the measurement of molar volumes of liquids at a given temperature when different liquids have the same surface tension. Mathematically

[P] = M1/4 g/D

Where [P] is the parachor, M is the molar mass of the liquid, g is the surface tension and D is the density of the liquid.

·         The value of Parachor is both additive and constitutive, i.e. its value depends upon the atoms present called Atomic Parachor and also on the structural units called Structural Parachor. With the help of the two, the total parachor value of a compound can be calculated. Thus Parachor is useful in deciding the chemical constitution of a compound.

·         Surface tension of a liquid is decreased by rise in temperature due to decrease in the strength of intermolecular attractive forces with rise in temperature.

·         Substances like soaps, detergents, coal, catalysts, activated charcoal, clays, antibiotics, bactericides and certain dyestuffs change the surface tension of a liquid and are called Surface Active Agents or Surfactants. Detergents when added to water, decrease its surface tension due to which its wetting ability is increased.

·         In case of solutions, surface tension is directly proportional to concentration.

·         Different methods for the determination of surface tension include.

·         Capillary Rise Method

·         Torsion Balance Method

·         Stalagmometer Method or Drop Method

·         Maximum Bubble Pressure Method

 

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