CH122_Lab_Experiment3 PDF

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Lab exam for Experiment 3...

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CH 122 LABORATORY TEST EXPERIMENT #3

VISCOMETRY

NAME: INSTRUCTOR'S NAME: SECTION NUMBER:

DATE:

Safety: Please wear safety glasses throughout this experiment. OBJECTIVES To measure the time of flow of distilled water and ethanol-water mixtures, and to determine β, the viscometer constant and concentration of an unknown sample. APPARATUS Cannon-Fenske Viscometer, glass tank, stirring plate with magnetic stirring bar, thermometer, stopwatch, 5 mL pipette. CHEMICALS Distilled water and ethanol-water mixtures.

EXPERIMENT # 3

VISCOMETRY INTRODUCTION It is a general property of fluids (liquids and gases) that an applied shearing force that produces flow in the fluid is resisted by a force that is proportional to the gradient of flow velocity in the fluid. This phenomenon is known as viscosity. The viscosity is measured in poise or centipoise and a viscosity of one poise is defined as a force of 1 dyne/cm2 that causes two parallel adjacent fluid layers 1 cm apart to move past each other at a velocity of 1 cm/s. In this experiment you will be asked to: (1) Measure the viscosities of distilled water and ethanol-water mixtures, and to determine the concentration of an unknown sample. A Cannon-Fenske Routine Viscometer and constant temperature water bath will be used. For thin liquids, viscosity is conveniently measured by determining the time of flow of a given volume of the liquid through a capillary tube under the influence of gravity. (See Figure 1.) If the same viscometer and volume of liquid are used each time, the capillary diameter and pressure gradient experience by the liquid as it flows from the upper reservoir will then be constant, and can be combined into a common term β, viscometer constant. It can be shown that the viscosity of a liquid, η, can be determined by the following equation: η=βxtxρ

(Equation 1)

where: β is the viscometer constant, t is the time of flow, ρ is the density of the liquid. For thick liquids, viscosity is usually determined by a Brookfield viscometer. Its principle of operation is to drive a spindle, which is immersed in the test fluid, through a calibrated spring. The viscous drag of the fluid against the spindle is measured by the spring deflection. Spring deflection is measured with a rotary transducer. The amount of deflection depends on the viscosity of the fluid, the rotational speed of the spindle, the size and shape of the spindle, the size and shape of the container used, and also the distance between the container wall and the spindle surface.

PROCEDURE PART A: Cannon-Fenske Routine Viscometer

Figure 3-1 - Cannon-Fenske Viscometer

(a) Operation and Calibration of Viscometer Discard any liquid in the viscometer, rinse and drain the viscometer several times with distilled water before proceeding. Carefully fit the viscometer into the special holder and make sure the viscometer is vertical and the water level in the bath is above the top red line of the viscometer. Add exactly 5mL of distilled water to the viscometer by using the 5 mL volumetric pipette provided and several minutes to pass before taking the time of flow measurement. Attach a rubber bulb to the end of the glass tubing that is attached by a rubber hose to the smaller tube of the viscometer and draw the water well above the upper red line. Please be careful NOT TO DRAW any water into the hose or rubber bulb. Remove the bulb and allow the liquid to flow freely from the upper to the lower reservoir. Measure the time required for the water to pass between the two red lines, above and below the lower reservoir.

Repeat this procedure three times with distilled water until your results agree to within 0.2 seconds with each other, then record the times in seconds on the data table. Measure and record the water bath temperature. Calculate β by using equation 1. The density of water at various temperature is given in Table 2. The viscosity of water, η, at various temperature is given in Table 1.

(b) Preparation of Sample and Calibration Graph Using an electronic balance, prepare four different 25.0 g samples of ethanol-water mixture consisting of 8, 16, 24 and 32% ethanol by weight in water. Prepare one mixture at a time. Use a dry 50 mL beaker, tare the weight of the beaker to zero. Then add 2 g of ethanol and 23.0 g of distilled water and mix the solution well. This would give you a 8% ethanol solution. For the 16% solution, use 4 g of ethanol and 21 g of water and so on. Drain the viscometer of any solution and condition it with about 5 ml of the sample, making sure no sample is left in the viscometer at the end. Then add exactly 5 ml of mixture to the viscometer using the 5 mL transfer pipette provided. Measure and record the times of flow as in part (a) three times for each mixture. Plot a calibration graph of time of flow vs ethanol by weight. Obtain an unknown pre-mix ethanol-water sample and make sure you record the unknown sample # on the data table. Each student must obtain his/her own unknown sample. Repeat the experiment with the unknown sample making sure you only use 5 mL of sample. Measure the time of flow three times. Determine the % ethanol by weight in your unknown sample from your calibration graph and record this value on your data sheet. At the end of the laboratory exercise discard any liquid sample left in all your glassware and rinse them out with distilled water. Any beaker or flask should be left upside down. Make sure your work area is left neat and tidy.

EXPERIMENT # 3 VISCOMETRY

DATE: / / D M Y

NAME:

PART A: Cannon-Fenske Routine Viscometer 1. Calibration of Viscometer The value of β in equation 1 is not needed in your determination of % ethanol in your unknown sample but it is used as a check on your technique.

Time of flow for distilled water Trial 1

s

Trial 2

s

Trial 3

s

Average

s

Temperature of Water ..............................................................................................

o

C

Density of Water, ρ (refer to Table #2)....................................................................

g/mL

Theoretical Viscosity of Water, η .............................................................................. (refer to Table #1)

cP

Viscometer Constant, β (see Eq 1).......................................................................... Please show your calculations below:

cPcm3/sg

2. Calibration Graph

Mass of Ethanol (grams)

Mass of Water (grams)

% Ethanol by weight

Time of Flow in seconds Trial #1 Trial #2

0.0

25

0

from part 1

2.0

23

8

4.0

21

16

6.0

19

24

8.0

17

32

Unknown Sample # ______

% ethanol by weight from graph =

%

Average Time of flow in seconds

EXPERIMENT #3 – Table#1 Viscosity of Water at Various Temperatures

Temp (oC)

Viscosity (cP)

Temp (oC)

Viscosity (cP)

15

1.1404

21

0.9810

16

1.1111

22

0.9579

17

1.0828

23

0.9358

18

1.0559

24

0.9142

19

1.0299

25

0.8904

20

1.0050

EXPERIMENT #3 – Table#2 Density of Water at Various Temperature

Temp. (oC)

Sp. Gr. of H2O

Temp. (oC)

Sp. Gr. of H2O

15

0.9991

21

0.9980

16

0.9989

22

0.9978

17

0.9987

23

0.9975

18

0.9986

24

0.9973

19

0.9984

25

0.9970

20

0.9982

26

0.9968...