AP Chemistry Unit 1 Lab
Density of a Solid and Organic Liquids
Introduction
All states of matter, gases, solids, and liquids exhibit the property of
density. Density is defined as mass per unit volume. Densities of most
substances change with an increase or reduction in temperature. Water
gm
for example at 4 C is most dense (1.00000
). The density of water at
ml
gm
gm
25 C is .9970
. The density of ice at 0 C is .9168
. Remember
ml
ml
1 ml = 1cm3 . At varying pressures, the density of liquids and solids is
relatively constant. For the case of a gas, the density increases
enormously as the pressure increases. Lithium has the low density of
gm
gm
.534
while Osmium has a density of 22.5
. Note here the relation
ml
ml
as the atomic mass increases then so does the density.
To measure the density of a solid, divide the measured mass by the
measured volume. To find the density of a liquid, perform the same
calculation as for a solid. The mass can be measured using a balance
while the volume can be measured using a pipette or graduated cylinder.
Precision and accuracy play a big role in the outcome of this
experiment. Consider the following targets after being shot with a rifle.
Precise but not accurate
Accurate but not precise
The above picture demonstrates the concept of accuracy and precision.
See your text for a more elaborate definition of these to terms. To
understand the concept of precision, consider the following set of data.
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Experiment
1
2
3
4
5
Measured Density
0.98 gm/ml
0.96 gm/ml
0.99 gm/ml
0.95 gm/ml
0.97 gm/ml
Average = 4.85/5 = 0.97
Absolute Deviation
0.01
0.01
0.02
0.02
0.00
Average Deviation
= 0.06/5 = 0.01
gm
0.01. Disregard any negative
ml
sign in the calculation of deviation or average deviation.
So then we can say the density is 0.97
Table of Known Densities – gm/ml
Pentane ............................................................. 0.626
Hexane .............................................................. 0.659
tert-Butyl Methyl Ether ....................................... 0.741
Toluene ............................................................. 0.867
Ethyl alcohol ...................................................... 0.785
Ethyl Acetate ..................................................... 0.902
Acetone ............................................................. 0.791
Carbon Tetrachloride ........................................ 1.594
Dichloromethane ............................................... 1.325
Materials for this lab
Milligram Balance
1 ml Syringe – graduated
a “TC” or “TD” pipette
2 sample vials and caps
2 cm length of rubber tubing
Graduated Cylinder
Forceps
Safety Considerations
Ware safety goggles
Ware a lab apron
All reagents used in this experiment are combustible. Do not use fire near
them.
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Pre Lab Questions
1.
In the introduction of this experiment, how many significant figures
is the density of water given at 4 C and at 25C?
2.
A one-liter flask is placed on a one pan of a very accurate balance.
On the other pan brass weights are placed such that both pans
balance exactly. This entire apparatus is placed in a complete
vacuum. The balance is tipped in favor of the flask. Is seems to
weigh more. Explain why.
3.
Determine the relative error if the experimental density is
gm
gm
measured at: .735
and the actual density is .750
.
ml
ml
4.
What does average deviation of a set of data tell us?
Procedure
Solid
1.
2.
3.
Liquids
1.
2.
3.
4.
Measure the mass of the unknown solid and record in the report
sheet.
Pour a specified volume of water in a Graduated cylinder. Use a
cylinder large enough so that the metal object will easily pass into
it.
Insert the metal object and record the volume change.
Examine the pipette carefully. Two types are most commonly used.
One will be labeled “TC” at the top (to contain) and “TD” at the
bottom (to deliver). The “TD” pipette delivers 1.00 ml from the top
graduation to the bottom of the pipette whereas the “TC” pipette
delivers 1.00 ml from the top graduation to the bottom graduation
marked “0”. Either pipette will work. Be sure to familiarize yourself
with use of the pipette.
Attach a 1 ml syringe to the pipette using a 2 inch section of a gum
rubber tube. Determine the masses of the solid metal object
supplied by the teacher and 2 labeled and caped vials. Be sure
your measure the masses to the nearest .001 gms.
Using the syringe as a pump, draw liquid into the pipette. Try to get
the liquid near to the top graduation. Record the amount of liquid in
the pipette within .001 ml. The last figure will be an estimate.
Expel a portion of the liquid from the pipette into the vial. Again
record the volume reading from the pipette.
Determine the masses of the liquids and vials to within .001
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Report Sheet
Unknown solid
Mass of Solid
__________________
Final Volume of graduated cylinder
__________________
Initial volume of the graduated cylinder
__________________
Volume of the solid
__________________
Known Liquid
Name of known liquid
_______________________
Mass of empty Graduated Cylinder
____________
Mass of Graduated Cylinder and liquid
____________
Mass of liquid
____________
Volume of liquid
____________
Known Liquid
Name of known liquid
_______________________
Mass of empty Graduated Cylinder
____________
Mass of Graduated Cylinder and liquid
____________
Mass of liquid
____________
Volume of liquid
____________
Calculations
Calculate the density of the solid
Compare your calculated density to densities of known solids using the
CRC handbook. Using the stated density in the CRC handbook perform
an error analysis.
Calculate the density of the liquid 1
Using the table provided in this experiment, calculate an error analysis.
Calculate the density of the liquid 2
4
Using the table provided in this experiment, calculate an error analysis.
Post Lab Questions
1.
Explain why there was error in your experiment.
2.
Most of your error was probably in measuring the volume used in
the calculation of density for the solid. What way could be used to
measure the volume of the solid?
3.
Can the determination of density be used to identify a material?
Why or why not?
4.
For the case of the rifle targets shown in the introduction, what
would the target look like with high accuracy and high precision?
Explain.
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