# The Viscosity of Liquids

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39. The Viscosity of Liquids

After studying the present lecture, you will be able to

Define viscosity and viscosity coefficient
Outline the method to measure viscosity using Ostwald viscometer
Determine the average molecular weight of a polymer

Determine the surface concentration of 1-butanol in aqueous solution

Measure the distribution coefficient of a solute betweenn two solvents

39.1 Introduction

Viscosity, one of the transport properties, arises because of intermolecular attractive and relatively long-range forces. Viscosity coefficient ([pic]), a specific constant characteristic of a liquid could be expressed by the following equation of Poiseuille.

[pic]
The constant a is sensitive to the shape and varies from zero for hard sphere or 0.5 for random coils to 2.0 for rigid rods.

Variation of specific viscosity of a solution [pic] (specific viscosity or reduced viscosity) is defined as [pic], where [pic] is the viscosity of solution i and [pic] the viscosity of pure solvent) with the concentration of solute is given by the equation:

[pic] (39.7)

where, c is concentration of the polymer in g/mL and K is a constant.

The determination of average molecular weight of polyvinyl acetate in acetone and methanol uses the following Procedure:

Prepare solution of polyvinyl acetate in acetone containing approximately 1 mg/ mL. From this prepare four more solutions of 0.2, 0.4, 0.6, 0.8 mg/ mL by dilution. Measure the specific viscosity of all the four solutions as described above.

Repeat the above experiment with solution of polyvinyl acetate in methanol.

Plot [pic] versus c and calculate [pic] and K from equation (2). Also, using the empirical formula (1) and constant of K and a from the table given below, determine the molecular weight of polyvinyl acetate in these two solvents.

|Solvent |K |a |
|Acetone |21.4 x 105 |0.68 |
|Methanol |38 x 10-5 |0.59 |

39.4 Excess Surface Energy