cHEMISTRY - Ii

Experiment 4

Aim of the Experiment

To determine the viscosity of Ethyl acetate wrt water by Ostwald's viscometer.

Principle

Since pressure is proportional to density of the liquid (P = hdg), the equation (ii) can be written as

12= d1^t1d2^t1 ……………….. (iii)

The absolute viscosity of the given liquid (η ₁) is obtained by substituting the value of viscosity coefficient of water (η ₂) in equation (iii).

Ostwald Viscometer

This is an apparatus commonly used for the determination of relative viscosity of a liquid.

Materials required

Apparatus

Ostwald viscometer, pipette, water bath, beaker, a piece of rubber tubing with a screw clip and stop watch.

Chemicals

Distilled water, unknown liquid and acetone.

Procedure

Clean the viscometer with distilled water and rinse it with acetone. Dry it by passing hot air.

1. Clean a piece of rubber tubing and attach it to the left limb of the viscometer.

2. Clamp the viscometer vertically in a water – bath so that mark A is below the surface of water.

3. Add 10 – 15 mL of distilled water into the right hand limb of the viscometer with the help of pipette (see that the lower bulb of the viscometer is half-filled). Allow the water to attain room temperature for 5 – 10 minutes.

4. Suck up the liquid by means of the rubber tubing till it reaches a little above the mark A. release it to flow down under the influence of gravity.

5. When the liquid just crosses the mark A, start the stop-watch and stop it as soon as liquid just crosses the mark B.

6. The difference between the final and the initial time of stop-watch gives the time of flow (t₂) of water.

7. Repeat the process at least four times so as to get a constant flow-time for water. Now repeat the entire experiment for the given liquid.

8. Empty the viscometer and dry it by mean of acetone. Clamp it vertically in a water-bath as done earlier.

9. Add 10 – 15 mL of given liquid into the viscometer and allow it to attain room temperature for 5 – 10 minutes.

10. Suck up the liquid by means of the rubber tubing till it reaches a little above the mark A. Note the time required by the liquid to flow from mark A to mark B which gives the time of flow(t₁) of given liquid.

11. Repeat the process at least four times so as to get a constant flow-time for water.

12. Calculate the relative and absolute viscosity of the liquid.

Observation table

Temperature of water – bath _______ºC

Calculations

1. Relative viscosity= η ₁/ η ₂ = d₁ t₁ / d₂t₂ = ___________________

Where = η ₁,d_1, t₁ are the viscosity coefficient, density and flow time of liquid and η _2, d_2,t₂ are the viscosity coefficient, density and flow time of water.

2. Absolute viscosity= η ₁ = d₁ t₁ η ₂ / d₂t₂ = ______________________

Result

1. The relative viscosity at ________⁰C = __________________ poise.

2. The absolute viscosity of the given unknown liquid at _____⁰C=_____________poise.

Questions

1. Can Oswald's Viscometer be used to measure the Viscosity of a very viscous liquid?

2. What is the difference between viscosity and density?