b.
Experiment: Study the variation of surface tension of detergent solutions with
concentration.
Principle: The surface tension of two liquid constituting mixtures do not differ greatly, the
surface tension of the mixture varies directly with the composition. Thus when the surface
tension of the mixture is plotted against composition a linear curve is obtained. The
concentration of the unknown mixture can be determined by using the Stalagmometer by drop
weight method using the relation.
γ1/γ2 = m1/m2
Requirements: Stalagmometer, detergent, weighing bottle, distilled water
Procedure: Detergent solution of different composition say 10%, 20%, 30%, 40% etc. by
volume are prepared. The Stalagmometer is dried and cleaned and is filled with distilled water.
The weighing bottle is placed below the tip of the Stalagmometer and a fixed number of drops
are allowed to fall in it. The weight of the weighing bottle and water is determined. From this
mass of the drop of water is found. Now the Stalagmometer is cleaned, dried and filled with
detergent solution of known concentration (10%) and the process is repeated as above. Knowing
the surface tension of the water at room temperature (γ1), the surface tension of the 10% solution
can be calculated by:
γ1 = γ2 m1/m2
Surface tension of the detergent solution of different known concentration is found out in similar
fashion. A plot is drawn with concentration along x-axis and surface tension along yaxis to get
the curve.
Result: From the curve obtained, concentration of known solution can be determined.
2. Viscosity measurement using Ostwald’s viscometer.
a. Experiment: Determination of viscosity of aqueous solutions of (i) polymer (ii) ethanol
and (iii) sugar at room temperature.
Theory: The co-efficient of viscosity (η2) of given aqueous solution is determined by making
use of the following formula:
η1/η2 = ρ1t1/ρ2t2 Where ρ1
= density of water at room temperature ρ2= density of
given aqueous solution at room temperature η1 = viscosity
of water at room temperature
Also t1 and t2 are the times of flow of water and the given aqueous solution for the same volume
of the two liquids through the fine capillary.
Requirements: Viscometer, stopwatch, beaker, clamp, pipette, a long rubber tubing, distilled
water, specific gravity bottle and thermometer.
Procedure: Clean the Ostwald viscometer thoroughly first with chromic
acid and then with running tap water and finally with distilled water. Rinse
it with acetone or ether and dry it with hot air from hot-air blower. It is
then mounted vertically and a definite volume of water is introduced into
bulb B and is sucked into A, above the mark P. The water then flows down
the capillary and the time t2, required to fall from mark P to Q is noted
accurately using a stopwatch. A mean of three observations is recorded.
The experiment is repeated with given liquid and the corresponding time
t1 is also noted.
Fig. Ostwald’s viscometer
Observation:
Room temperature = t0C
Density of water at t0C, ρ1 = ……..
Viscosity of water at t0C, η1 = …….. Poise
Water Unknown liquid
Sl. No. Time of flow in Mean value Time of flow in Mean value
seconds seconds
1. 1.
2. t2= 2. t1=
3. 3.
4. 4.
Observation and calculations:
Weight of empty weighing bottle = w1 g
Weight of weighing bottle + water = w2 g
Weight of weighing bottle + liquid = w3 g
Weight of water = w2- w1 g
Weight unknown liquid = w3- w1 g
Surface tension of water = γ2 dynes /cm
Surface tension of liquid = γ1 γ2
ρ1/ ρ2 = (w3- w1/ w2- w1)
The Co-efficient of viscosity of the liquid is given by
η1= ρ1t1/ρ2t2 x η2
The relative viscosity of the liquid is given by
η1/η2 = ρ1t1/ρ2t2
By putting the respective values, the relative viscosity of liquid is calculated
η1= (w3- w1) t1/ (w2- w1) t2 x η2
Thus η1 can be calculated
Result: The Co-efficient of viscosity of the liquid is ……… poise
b. Experiment: Study the variation of viscosity of sucrose solution with the concentration
of solute.
Requirements: Sucrose, viscometer, stopwatch, beaker, clamp, pipette, a long rubber tubing,
distilled water, specific gravity bottle and thermometer.
Theory: The co-efficient of viscosity(η2) of given aqueous solution is determined by making
use of the following formula:
η1/η2 = ρ1t1/ρ2t2
Where ρ1 = density of water at room temperature
ρ2 = density of given aqueous solution at room temperature η1 = viscosity of
water at room temperature
Also t1 and t2 are the times of flow of water and the given
aqueous solution for the same volume of the two liquids
through the fine capillary.
Procedure: A rubber tube is attached to a cleaned and
dried viscometer and it is clamped to a stand. 15 ml of water
is introduced into the larger bulb B of the viscometer. It is
then sucked into the bulb till it rises to the mark P. The
time of flow of water, t1, from mark P to Q is noted by
using a stopwatch. Fig. Ostwald’s viscometer
The viscometer is then dried. A series of aqueous solution of sucrose of different compositions
i.e. viscosity of each sucrose solution is determined and a graph plotted between viscosity and
composition. From the graph, the composition corresponding to the viscosity of sucrose solution
with unknown composition can be determined.
Observation and calculations:
Room temperature = t0C
Time of flow of the solvent = ………., Mean = t2
Density of pure solvent = ρ2
For solutions:
Flow time Density (ρ) η
Concentrations (i) (ii) (iii) Mean (t sec)
2%
4%
6%
8%
10%
A graph is plotted between concentration of sucrose along x axis and corresponding viscosity
along y axis. From the shape of curve obtained, the variation with concentration can be studied.
From the plot the composition corresponding to the viscosity/ time of flow of unknown liquid
can be determined.
3. pH metry
a. Experiment: Study the effect on pH of addition of HCl/NaOH to solutions of acetic
acid, sodium acetate and their mixtures.
Theory: Acetic acid is a weak acid, it dissociates to a very small extent.
CH3COOH = CH3COO- + H+
When a few drops of HCl solution is added to acetic acid solution, there will be increase in H +
ion concentration, thus the above equilibrium shift towards left. There is suppression in the
degree of ionization of acetic acid. Further addition of HCl to acetic acid solution will
continuously increase H+ concentration and pH of the solution will decrease continuously.
When a few drops of strong base i.e. NaOH are added, the OH - ions added are neutralized by
the acetic acid. There will be very little change in the pH. Also some CH 3COOH-CH3COONa
buffer formed, which resist some change in pH to a certain level .But further addition of NaOH
solution will continuously increase the pH of the solution.
Requirements: 1.0 M HCl, 1.0 M NaOH, 1.0 M CH3COOH, 1.0 M CH3COONa, pH meter,
buffer
Solutions (pH = 4 and 9), distilled water, thermometer, beakers, pipette, burette.
