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CHEM 0011
Experiment 3 – Introduction to Weighing Technique and Density
Determination
Objectives
1. To learn to use an analytical balance.
2. To learn a proper weighing technique – weighing by taring.
3. To use the weighing technique to determine the density of water, ethanol
and gylcerol.
Apparatus:
1. 3- 50 mL Erlenmeyer flask and stopper
2. 10 mL graduated cylinder
3. Analytical Balance
Solid:
1. Sand (for instructor demonstration)
Liquids:
1. Distilled water
2. Ethanol (also named ethyl alcohol)
3. Glycerol (also named glycerine)
Introduction
The Balance Room
Balances are sensitive
to drafts, changes in
temperature, or the
vibrations caused by
moving people. The
balances are stored in
a separate room to
minimize these
variables and are
placed on concrete
tables.
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Balances are very expensive and are sensitive to attack by corrosive chemicals.
Do not transfer liquids in the balance room. When possible, chemicals should
be added to the weighing container outside of the balance chamber. During the
weighing process, the weighing container should be placed on a clean surface,
such as a kimwipe, so that the bottom of the container does not pick up any dust.
It is important that you clean up all chemical spills. If in doubt consult your
instructor.
Weigh boats are disposable containers used for weighing. They are made of
polypropylene plastic and are inexpensive. A used weigh boat should be
discarded in the waste container. All chemicals and spatulas that are used should
be returned to their proper places. Depending on the experiment, other types of
weighing container could be a porcelain crucible, an aluminum container or a
small beaker.
The balance room must be kept tidy. Materials taken into the balance room
include datasheets, pen and the sample to be weighed. Enter mass
measurements directly on the datasheet with your pen. Before you leave your
balance, make sure:
1. The balance and the area around it is clean. Spills inside the balance
should be brushed off using the brush on top of the balance. Spills on the
concrete table should be cleaned using Kimwipes.
2. Close all the doors of the balance.
3. Turn off the balance.
Practice Question – to be done before lab
For a material to float on the surface of water, the material must have a density less than
that of water (1.0 g/mL) and must not react with the water or dissolve in it. A spherical ball
has a radius of 0.5000 cm and weighs 2.00 g. Will the ball float or sink when placed in
water? (Volume of a sphere = 4/3 r3)
Space next page for practice.
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Part A - Technique on Weighing by Taring
Introduction
There are many different types of balances or
scales available to measure the mass of a sample.
The selection of the balance depends on the mass
of the object or sample and the precision needed
for the measurement. A top-loading balance is
used to determine the approximate mass of the
sample needed. In this course, we will be mostly
using electronic analytical balances. These
balances are easy to operate and are capable of
measuring to 0.0001 g. Models of these balances
vary in the labs. Consult your instructor and
operating manual specific to the model of the
balance.
The focus in this course will the technique of weighing by taring. This technique
is used when the mass of the empty container is not important. The empty
container is ‘zeroed’ or tared on the balance.
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Part A - Instructor Demonstration of Weighing by Taring
Procedure:
1. Place a weigh boat on the balance pan. Close the doors and wait for the
reading to stabilize. Press briefly on the control bar or the tare button and the
display changes to 0.0000 g. The weight of the weighing boat is now tared.
2. Remove the weigh boat from the balance and set it on a piece of kimwipe.
With a spatula, carefully add the sample to the weigh boat. Place the weigh
boat back on the balance pan. Close the doors and wait for the reading to
stabilize. Record the mass of the sample.
Part A - Datasheet for Weighing by taring
1. Mass of weigh boat and sample
______________________ g
2. Mass of empty weigh boat
3. Mass of sample
0.0000 g
______________________ g
Question:
1. How many significant figures does the mass of the sample have?
2. How many significant figures does the mass of the tared empty weigh boat
have?
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Part B – Density Determination
Introduction
Density is a physical property, which can be used to identify substances. Density
is defined as the mass per unit of volume.
Density = mass/volume
Mass is a measure of the quantity of matter contained in an object. Volume is a
measure of the space occupied by an object.
Part B: Density Determination (to be carried out with a lab partner)
Procedure
1. Add approximately 10 mL of distilled water with a 10 mL graduated
cylinder. Record the volume {see page 6).
2. Turn on the balance and tare a clean and dry 50 mL Erlenmeyer flask
with a rubber stopper on the balance. Do not turn off the balance.
3. Remove the tared Erlenmeyer flask from the balance. Outside the balance
room transfer approximately 8 to 9 mL of the distilled water from the
graduated cylinder to the Erlenmeyer flask outside the balance room. Put
the stopper on the Erlenmeyer flask to avoid evaporation. Record the
volume of water remaining in the graduated cylinder.
4. Return to the balance that was used in step 2, and weigh the Erlenmeyer
flask, distilled water and stopper. Record the mass of the Erlenmeyer
flask, distilled water, and stopper on the datasheet.
5. Turn off the balance.
6. Calculate the volume of the distilled water that was transferred into the
Erlenmeyer flask to the proper number of significant figures.
7. Use clean and dry 50 mL Erlenmeyer flask with a rubber stopper for each
liquid. Repeat the above steps 1 to 6 for ethanol and glycerol. Use the
labelled graduated cylinder beside the respective bottles of ethanol and
glycerol. Return the graduated cylinder to the correct bottle when done.
8. Dispose of chemicals in the appropriate container.
9. Record the room temperature on the datasheet.
Do not use water to clean the flasks that contain ethanol. The flasks can be airdried in the fumehood.
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Datasheet:
Part B: Density
Water:
1. Mass of Erlenmeyer flask + distilled water + stopper
_______________ g
2. Mass of empty Erlenmeyer flask + stopper
3. Mass of distilled water
0.0000 g
__________________ g
4. Volume of distilled water measured in graduated cylinder
___________ mL
5. Volume of distilled water remained in graduated cylinder ______________ mL
6. Volume of distilled water transferred
______________ mL
Ethanol:
1. Mass of Erlenmeyer flask + ethanol + stopper
2. Mass of empty Erlenmeyer flask + stopper
3. Mass of ethanol
_______________ g
0.0000 g
__________________ g
4. Volume of ethanol measured in graduated cylinder
___________ mL
5. Volume of ethanol remained in graduated cylinder
______________ mL
6. Volume of ethanol transferred
______________ mL
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Glycerol:
1. Mass of Erlenmeyer flask + glycerol + stopper
_______________ g
2. Mass of empty Erlenmeyer flask + stopper
3. Mass of glycerol
0.0000 g
______________________ g
4. Volume of glycerol measured in graduated cylinder
___________ mL
5. Volume of glycerol remained in graduated cylinder
______________ mL
6. Volume of glycerol transferred
______________ mL
Sample Density Calculation:
Show work here for Ethanol:
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Summary of Densities Measured:
Room Temperature = ___________________________________
Density of Liquid
Water
Ethanol
Glycerol
Percent Error Calculation
The percent error is defined mathematically by equation:
% Error = |accepted value - experimental value| * 100
accepted value
Given that the accepted value for the density of ethanol is 0.79 g/mL, calculate the % error in your
result. Show work here and report your answer with the proper number of significant figures.
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Questions:
1. Do you think temperature affects the density of the liquid. Explain your
answer.
2. A glass beaker weighs 48.462 g. A sample of 4.00 mL of antifreeze
solution is added, and the container plus the antifreeze weigh 54.51 g.
Calculate the density of the antifreeze solution.
CHEM 0011 – Lab 3 - 2011