Advance Kentucky
Mr. Acampora
AP Chemistry
Density Determination and Identification of
An Unknown Pure Liquid
Introduction
Chemists use many properties to identify and characterize a substance. In order to be useful, the property must be
independent of the amount of substance present in a sample. This distinction, of intensive vs. extensive properties, is important
to scientists because it allows them to describe intrinsic properties. For example, if you were considering the operating cost of a
car, you would be interested in knowing the fuel efficiency in terms of miles per gallon (or kilometers per liter). Clearly, the
amount of gasoline used by a car in a trip will depend on the length of the trip. Both distance and volume of gasoline consumed
are extensive properties. The ratio between these properties, however, is an intensive property that should remain (roughly)
constant for a particular car and can be used to characterize it.
One of the most important intensive properties of any sample of matter is its density, defined as its mass per unit volume. If
we were to consider two samples of the same substance, for example two blocks of pure Iron, we would expect that the block
with the greater volume would also have a greater mass, but that the ratio
Iron
of mass to volume would be constant, as it would for any sample of pure
Iron under the same conditions of temperature and pressure.
Density of Iron 
M1 M 2

V1 V2
Mass = M 1
Mass = M 2
Volume = V 1
Volume = V 2
Clearly, different substances would be expected to vary greatly in
their densities, depending on the mass, size, and arrangement of the
individual particles that make up the substance. However, the density of any sample of a given substance, measured under
identical conditions of temperature and pressure, would have a fixed value independent of the either the mass or the volume.
Density can be measured for all phases of matter--solids, liquid, and gases--and values range from approximately 0.00009 g/mL for
Hydrogen gas at STP to approximately 22.5 g/cm3 for Osmium metal under the same conditions. [Note that the density of a
substance can be expressed in different units. In the SI system, the base unit would be the kg/m3, however more common
laboratory units are the g/cm3 for solids or liquids, and the g/L for gases. Densities are rather confusingly expressed in the English
system because the ounce is both a unit of mass and a unit of volume, the fluid ounce. We will avoid this conundrum and restrict
our attentions to the metric system.]
On an atomic level, the value of the density of a particular substance depends on the mass and volume of the particles that
make up the substance, whether atoms, molecules, or ions, and the way that the particles are arranged. This relationship among
the particles of a substance clearly differs in different phases of matter, solids, liquids, and gases, and even within different
substances of the same phase. Thus, even though both Diamond and Graphite are composed of only Carbon atoms, the
arrangement of Carbon atoms in each substance differs and Diamond has a greater density than Graphite [Different forms of the
same element, for example Diamond and Graphite, or Diatomic Oxygen (O2) and Ozone (O3) are called allotropes . Clearly, they
have unrelated properties because the properties of a substance are determined as much by the forces between atoms as they are
by the atoms themselves.]
The density of even a pure sample of a given phase of substance depends on the conditions of temperature and pressure under
which the density determination is made. Though a sample’s mass will not be affected by temperature and pressure, the volume
may change, and hence the density will vary. Such changes can be dramatic, in the case of gases, or virtually insignificant, as is
the case for solids and liquids at ordinary temperatures and pressure. As an example, the density of liquid water varies from a
high of 1.000 g/mL at 4 C to a low of 0.962 g/mL at its boiling point of 100 C. In this lab, we will make all measurements at
normal laboratory conditions, which will be quite close to Standard Temperature and Pressure (STP, defined for most
measurements at 25 C and 1.00 atm), and our values should be quite close to accepted values.
As a matter of convention, volumes of gases and liquids are commonly expressed in Liters and milliliters (L and mL),
while volumes of solids are expressed as cubic centimeters (cm3). By definition, 1 mL = 1 cm3.
1
Experimental Overview
There are many ways in which the density can be determined. One easy and approximate method to
measure the density of liquids directly is to use a hygrometer, a calibrated, weighted tube that floats in the
liquid. Because the tube floats when the mass of the displaced fluid equals the mass of the hygrometer, the
tube will float higher in a liquid of greater density (since it will displace a smaller volume of liquid). A related
“gizmo” is sold by auto parts stores to test the density (and effectiveness) of radiator fluid. Since radiator fluid
is a mixture of Water and Ethylene Glycol, a more dense liquid, the density of the solution is a function of its
composition. The greater the percent Ethylene Glycol, the greater the density. An automobile “radiator fluid
tester” usually consists of a large syringe that can sample the fluid and that contains four or five colored
spheres of different densities. When the radiator fluid is drawn into the syringe, all spheres with a density less
than the density of the fluid will float. The more spheres that float on the fluid, the greater the density of the
fluid (which is related to its composition and its effectiveness in preventing overheating and/or freezing).
In our laboratory determination of density, two quantities must be measured--the mass and the volume of
a particular sample of matter. The mass can most easily be determined by weighing the sample on a balance.
[The quantity that we think of as "weight" is really the mass of the sample--the difference between an object's
"mass" (or quantity of matter) and its "weight" (or gravitational pull at Earth's surface) falls into the province of Hy grometer
physics and need not concern us so long as we remain close the surface of the earth. By comparing the weight of a sample with
the weight of known masses, we are actually determining the mass directly.]
The mass of a sample of liquid in a container can be found by taking the difference between the mass of the container plus
the liquid and the mass of the empty container. This procedure depends on the fact that mass is conserved and the masses of
objects are therefore additive (mass is
one of the few properties that exhibits
this "additivity"--the volume obtained
by mixing 50.00 mL of Water with
50.00 mL of Methanol will be
measurably less than 100.00 mL, but
the mass obtained by mixing 50.00 g of
Water with 50.00 g of Methanol will
indeed be 100.00 g). The mass of a
solid can be determined directly by
placing the solid on the pan of the
balance, but more commonly a sample
will be added to a weighed container,
and the container’s weight will be
subtracted from the combined mass.
50.00 mL
Methanol
50.00 mL
Water
< 100.00 mL
Mixture
The volume of a liquid can be determined by means of a calibrated container. In the laboratory, a
graduated cylinder is often used for routine, non-critical measurements of volume. Accurate measurement of
liquid volume can be made using a buret or a volumetric pipet, or in this case, using a pycnometer.
As in many laboratory procedures, the first step will be to calibrate the instrumentation. In this lab
exercise, this calibration will take the form of determining precisely the mass of a liquid of known density
(water) required to completely fill the pycnometer. To determine the volume of the pycnometer, you will use
the measured mass of the liquid and its known density to calculate quite accurately the volume of the
pycnometer. You will use distilled Water, whose density is known accurately as a function of temperature. As
usual, when you make any experimental measurement, be sure to record your data to the correct number of
significant figures and round your calculations appropriately.
Pycnometer
(Gay Lussac
Bottle
2
Density of Pure Water at Laboratory Temperatures
Temp (C)
15
16
17
18
19
20
21
22
23
24
25
Density (g/mL)
0.9992
0.9991
0.9989
0.9988
0.9986
0.9984
0.9982
0.9979
0.9976
0.9974
0.9970
In this experiment, you will determine the density of a pure liquid by the procedure outlined above. First, you will obtain
the weight of an empty, clean and dry pycnometer with stopper. The vessel will then be filled completely with distilled water,
and the vessel and its contents will be massed. From the difference in these masses, the mass of the water will be determined and
then, from the known density of water, the volume of the vessel will be calculated. The vessel will then be emptied and dried,
filled with a different liquid, and weighed again. From the mass of the liquid and the volume of the flask, the density of the
liquid can be determined quite precisely.
Experimental Procedure
A. Calibrating the Flask
1. Obtain a clean and dry plastic pipette, which we will use to substitute for a pycnometer. If the “pycnometer” is not clean
and dry, clean it with soap and water, rinse it with a few mL of Acetone, a fast drying liquid, and dry it by letting it stand a few
minutes in air or by gently blowing compressed air into it for a few moments. Do NOT blow into it with your breath, as
condensation of your exhaled water vapor will form on the inside surfaces of the vessel and will alter its mass. Weigh the dry
pycnometer with its stopper on the balance with an accuracy of ± 0.01 g. Fill the pycnometer with distilled water until the liquid
level is nearly to the top of the ground surface in the neck. Work the stopper into the flask carefully, letting the excess water out
of the capillary in the stopper and being sure that there are no air bubbles remaining in the flask. Wipe any water from the
outside of the pycnometer with an absorbent paper towel in order to soak up all excess water. Again, weigh the pycnometer,
which should be completely dry on the outside and full of water, to the nearest ±0.01 g. Measure the temperature of the
laboratory (and we will assume that the water is at the same temperature). Given the density of water at the temperature of the
laboratory (see above table) and the mass of water in the flask, you should be able to determine the volume of the flask to four
significant figures. Pour the distilled water into the sink.
B. Density of a Pure Liquid.
2. To make certain that the liquid in the pycnometer is not contaminated by residual water, it is good laboratory technique to
rinse the flask several times with a few mL of your liquid and discard the liquid into the appropriate waste container. Using the
same flask from part A, add a few mL of pure liquid to the flask, stopper and agitate the flask to make sure that the liquid
completely wets the interior surface of the flask and stopper, and discard the liquid into the appropriate waste container. Repeat
this rinsing, then fill the pycnometer with pure liquid as in part A. Stopper and dry the pycnometer as you did when working
with water and then weigh the filled and stoppered pycnometer, again making sure its surface is completely dry. This
measurement, used in conjunction with those made previously, will allow you to accurately determine the density of your liquid.
Analysis
A. Calibrating the Flask.
1.
3
Prepare a data table and record the following values. Be sure to include correct units and round your values
appropriately.

Mass of clean and dry flask

Mass of clean flask completely filled with distilled water

Temperature of room

Density of water at the room temperature
2.
Calculate the following values, showing your calculations, including units, and rounding your answers appropriately.

Mass of water in the flask

Volume of water (= volume of flask)
B. Density of a Pure Liquid.
3.
Again, show all of the raw data, neatly labeled, to the correct precision, and include units. Raw data includes:

Mass of clean pycnometer completely filled with liquid
4.
Using this datum and the measured and calculated values from part A, determine (with units and appropriate rounding):

Mass of liquid in the pycnometer.

Density of pure liquid.
5.
Obtain the identity of your liquid from your instructor. Use the information in the table below to determine

The Formula Mass of your substance, expressed in g.mol-1

The molar volume of your substance, expressed in mL.mol-1
6.
Perform an Error Analysis of your results.
 Determine the “accepted” density of your liquid at STP. The value can be obtained from many sources. You might
try the CRC Handbook or one of the catalogs for chemical suppliers, such as Aldrich Chemical Co.
 Calculate the percent error of the experiment
 If the percent error is greater than 1%, postulate the cause of the error. Be sure that any proposed source of error is
consistent with the +/- sign of the error.
Your Complete Laboratory Report will follow the attached rubric:

An introduction explaining the purpose of the lab (what you are trying to find) and an overview of the experimental
method you will employ (including independent and dependent variables, hypotheses, etc.).

The data tables showing all of your labeled experimental data and calculated data as outlined above. Be sure to show
your calculations and to round your results appropriately.

A conclusion in which you give the final values determined and compare them to accepted values that you may obtain
from reference material. Any significant error should be explained.
This laboratory experiment will encompass the following concepts:





The scientific method
Calibrations and repeatability / Accuracy and precision
Scientific writing
Density
Scientific Notation and Significant Figures
4
Pre-Laboratory Assignment
1.
a. What are the SI standard and common laboratory units for each of the following quantities?
i. Mass
ii. Volume
iii. Density
b. The density of Iron is given as 7870 kg.m-3. Express this value in g.cm-3.
c. Estimate the density of a Copper penny, including an appropriate metric unit. [Note, prior to 1982, United States pennies
were composed of almost pure copper. Due to the rise in the price of copper in the early 1980’s, the US mint began making
pennies out of a cheaper metal (zinc) with a fairly skimpy copper coating.]
2. A chemist finds two
unlabeled bottles of clear
liquids in the chemical
storage cabinet. Nearby, she
finds two labels specifying
the isomers 1-Propanol and
2-Propanol and giving their
accepted densities (see right).
The chemist has a flask of
volume 25 mL  1 mL and a
balance capable of reading to
 0.1 g.
H
H
H
H
|
|
|

H  C  C  C  O  H
|
|
|
H
H
H

H
O
|
|
H  C  C
|
H

|
H
a. What are isomers?
b. If the chemist fills
the flask completely with
each isomer (being careful
not to contaminate the
samples), what is the range of
possible masses consistent
with the uncertainty in the
volume of the flask?
1-Propanol
C3H8O
F.M. = 60.11 g/mol
D = 0.804 g/mL
2-Propanol
C3H8O
F.M. = 60.11 g/mol
D = 0.785 g/mL
c. Is it possible to identify which label belongs to which bottle given only the equipment listed above? Explain?
3. A student performs the lab exercise as described above and collects the following data:
Mass of empty pycnometer = 16.22 g
Mass of pycnometer + water @ 22 oC = 42.40 g
Mass of pycnometer + unknown liquid @ 22 oC = 37.53 g
Use this data to calculate
a. The volume of the pycnometer.
b. The density of the unknown liquid at 22 oC.
If a small air bubble is present, but unnoticed, when the pycnometer is filled with water, how will this affect
c. The experimental volume of the pycnometer.
d. The experimental density of the unknown liquid..
4. Prepare a list of all raw data measurements that you will make during the exercise.
5
H
|
C  H
|
H
Scientific Laboratory Report Rubric
Title of Report __________________________________
Author’s name: __________________________________
Beginning
1
Developing
2
Accomplished
3
Exemplary
4
Title &
Introduction
Does not give
any information
about what to
expect in the
report.
Title does not fit
the experiment
and the
introduction
gives very little
information.
Appropriate title,
introduction is
clear with a few
missing details.
Appropriate title
and introduction
presents a
concise lead-in
to the report.
Hypothesis
Does not
address an issue
related to the
topic.
Addresses an
issue, which is
unrelated.
Clearly
communicated
and addresses an
issue somewhat
related.
Clear, concise,
and addresses a
real issue
directly related
to proposed
research.
Materials
List not
provided.
List provides
some materials.
Complete list
provided.
Detailed list with
descriptions
provided.
Procedure
Not sequential,
most steps are
missing or are
confusing.
Some of the
steps are
understandable;
most are
confusing and
lack detail.
Most of the steps
are
understandable;
some lack detail
or are confusing.
Presents easy-tofollow steps that
are logical and
adequately
detailed.
Data & Results
Data table graph
and/or results
not provided.
Data table graph
and/or results are
missing
information and
are inaccurate.
6
Data table graph
and/or results are
generally
accurate.
Data table graph
digital photos
from actual lab
investigation
and/or results are
neatly completed
and totally
accurate.
Score
______ x 5
Total _____
Beginning
1
Developing
2
Accomplished
3
Exemplary
4
Presents an
illogical
explanation for
findings and
does not address
any of the
questions.
Presents an
illogical
explanation for
findings and
addresses few
questions. Does
not relate
findings to
topics studied in
class.
Presents a logical
explanation for
findings and
addresses most of
the questions.
Mostly connects
findings to topics
studied in class.
Presents a logical
explanation for
findings and
addresses all of
the questions.
Connects
findings to topics
studied in class.
Formal Writing
Style
Informal, 1st
person writing
style.
Some written in
3rd person formal
writing style
with several
lapses.
Mostly written in
3rd person formal
writing style with
a few lapses.
Written in 3rd
person formal
writing style.
Grammar &
Spelling
Very frequent
grammar and/or
spelling errors.
More than two
errors.
Only one or two
errors.
All grammar and
spelling are
correct.
Attractiveness
Illegible writing,
loose pages.
Legible writing.
Word-processed.
Word processed,
clean and neat,
illustrations
provided.
Timeliness
Report handed
in more than one
week late.
Up to one week
late.
Up to two days
late.
Report handed in
on time.
Sources
None listed
Textbook, only
2-4 Sources
4 or more
Sources
Analyses &
Conclusions
Score
_____ x 10
Total _____
Total
Total ÷ 24
7
Beginning
1
Developing
2
Accomplished
3
Exemplary
4
32/50 D
37/50 C
42/50 B
48/50 A
Score