DENSITY
Presented by
Dr / Osama Saher
Items to be discussed today:
Density
Definitions & Laws
Specific gravity
Specific volume
Experiments
Exp.I. Determination of the density of a
given liquid at a definite temperature
using pycnometer
Exp.II. Determination of the density of
solutions containing different
concentrations of sodium chloride at
room temperature
Density
It is the mass per unit volume at a definite
(Temp.) and (Pressure)
Density (ρ) = Mass/Volume
Units of Density:
1- In Metric (CGS) System: g/cm3 (g/ml)
N.B.:
 1 ml or 1 cm3 of water weighs 1 g at 4Co
 1 g is defined as the mass of 1 cm3 of water at
4oC (i.e Density of water = 1 g/ml at 4 co )
2- In English System: gr/fl ℥ - lb/gal
Specific gravity
It is the ratio of the weight of a substance to
the weight of an equal volume of another
substance chosen as a standard. Both
substances having the same temperature.
Water  Standard for liquids and solids
Hydrogen / Air  Standard for gases
Specific gravity =Ws / Ww = ρs / ρw
Density versus Specific Gravity
Density
Specific Gravity
1. Concrete number
1. Abstract number (no
(has unit)
unit), being a ratio
2. Constant value (at
2. Vary in value with
constant temp.)
the systems of
measure
e.g. for water: Sp. gravity
e.g. for water: 1 g/ml,
=1
455 gr/fl ℥ , 62.5 lb/ft3
N.B.:
• Density = Specific Gravity (for value) in Metric
system only not in English system
• Standard temp. used in Sp. Gravity = 25oC for all
substances, except alcohols (15.56oC)
Volume
• It is expressed in terms of liter, ml or cubic
•
centimeter (cm3).
The volume of l g of water at 1 atmospheric
pressure and 4°C was equivalent to 1 cm3.
(One liter = 1000.027 cm3).
Specific volume
It is the volume occupied by 1 g of a substance
at a given temp. and pressure (reciprocal of
density)
V = 1/ρ (cm3/g)
Exp.I. Determination of the density of a given liquid at
a definite temperature using pycnometer
 Specific gravity bottle
(pycnometer) is made of
glass, with 10, 25 and
100 ml capacity.
 A capillary stopper is
fitted into its mouth to
allow the removal of
excess liquid and obtain
accurate volume.
Capillary tube
Procedure:
1. Clean the pycnometer by rinsing with water.
2. Weigh the stoppered dry pycnometer.
3. Fill the pycnometer with purified water and place
the stopper. (Small amount of water comes out
the capillary, wipe out the excess liquid outside
the tube using tissue paper).
4. Weigh the pycnometer with water.
5. After removing the water, rinse the pycnometer
with the test liquid (5 ml) and fill the liquid in the
pycnometer in the same way.
6. Weigh the pycnometer with the test liquid.
7. Repeat each step at least three times and report
the average.
Empty pycnometer
(W1)
Water filled pycnometer
(W2)
Liquid filled pycnometer
(rinse with 5ml first)
(W3)
Observations:
1. Volume of pycnometer (V)
2. Weight of empty, dry pycnometer with stopper
3.
4.
(W1)
Weight of pycnometer with water (W2)
Weight of pycnometer with test liquid (W3)
Calculations:
1. Weight of water in pycnometer (W2 - W1)
2. Weight of test liquid in pycnometer (W3 - W1)
3. Density of liquid
Density of liquid
accurate volume of pyc. is known
Weight of liquid (W3 – W1)
Ρ (liq) =
——————————
Volume of liquid (V)
accurate volume of pyc. is unknown
Weight of liquid (W3 – W1)
Ρ (liq) =
——————————
Weight of water (W2 – W1)
Sp. Gravity
X
Ρ (water)
Example:
A specific gravity bottle weighs 23.66 g. When
filled with water it weighs 72.95 g; when filled
with another liquid it weighs 73.56 g. What is
the specific gravity of the liquid?
Solution:
73.56 g - 23.66 g = 49.90 g of liquid
72.95 g - 23.66 g = 49.29 g of water
Specific gravity of liquid = 49.90 (g)/49.29 (g) =
1.012
Specific gravity of liq.
= 1.012 g/cm3
Density of liq. =
X Density of water
Exp.II. Determination of the density of solutions
containing different concentrations of sodium chloride
at room temperature
1% w/v
2% w/v
4% w/v
10% w/v
Unknown
1) To study the effect of added salt on density
2) To determine the concentration of unknown solution after
determination of its density.
Procedure:
1. Follow the procedure in experiment no.1.
2. Determine the density of the different sodium
chloride solutions (1-10%) as described
before.
3. Plot a graph between the concentration of
sodium chloride solutions and density.
Comment on the plot concerning the effect of
concentration on density.
4. On the basis of the graph, determine the
concentration of an unknown sodium
chloride solution after determining its
density.
Observations:
1. Temperature (room temperature) = …… °C
2. Weight of pycnometer at room temperature
(W1) = …… g
3. Weight of pycnometer + water (W2) = …… g
4. Weight of pycnometer + solutions of different
concentrations of NaCl in water = (Wc).
(Use table A to record your data).
5. Calculate the density of different NaCl
solutions. Tabulate your results in table B.
Results:
Table(A)
Replicates
Weight (Wc) (g)
Concentration of sodium chloride in water % w/v
0
a
b
c
1
2
4
10
unknown
Results:
Table(B)
Sample.
Number
Conc. of NaCI
in water
(% w/v)
Weight of
Solutions (Ws).
Replicates
a
1
0
2
1
3
2
4
4
5
10
6
unknown
b
c
Average
weight
(a+b+c) /3
Density (Calcula
ted)
(Ws / Ww) x ρw
Results:
• Increasing conc. of NaCl will increase the density
•
of the solution.
From the graph, we can determine the conc. of
unknown solution of NaCl after determination of
its density.
Density
(g/ml)
Intercept at value 1
(water density)
Nacl Conc.
(%w/v)
General Comment on Graphs
1. What is drawn on Y-axis (dependant variable)
and on X-axis (independant variable)
2. Shape of resulting curve
3. Slope and any special points
4. What to be concluded from this shape, slope,
special points
So, Let’s apply this on our curve
PRECAUTIONS
Clean and dry Pycnometer.
Same Pycnometer for both water and liquid, if
broken????????.
Capillary Stopper should be fitted inside the
Pycnometer.
The Capillary should be directed upwards, not
downwards.
Drain the excess coming out the capillary with a
tissue paper.
No air bubbles inside the capillary or pyc. (if
any, refill and close pycnometer again and drain
the excess).