CHEM 121 Highline Community College Introduction to Density Questions: Chemistry Answers 2021

CHEM 121 Highline Community College Introduction to Density Questions: Chemistry Answers 2021

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CHEM 121 Highline Community College Introduction to Density Questions

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CHEM 121: Density
Introduction
to Density
We commonly use the terms ?heavy? and ?light? to describe various objects. For
example, a thick textbook might be considered ?heavy as a rock? while tissue paper
might be considered ?light as a feather.? These terms, ?heavy? and ?light?, actually
convey the relative density of a substance.
In science, the densities of different substances can be compared to learn more about
their masses or the arrangement of atoms in the substance. For example, a cast-iron
skillet is heavier than an aluminum skillet of the same size, so assuming the atoms are
packed similarly in both solids, one can conclude that iron atoms must have a greater
mass than aluminum atoms. Using the Periodic Table one can see iron?s atomic mass is
55.85 amu while aluminum?s atomic mass is 26.98 amu, confirming the prediction that
iron atoms have a higher mass than aluminum atoms.
Mathematically, density is defined as a substance?s mass-to-volume ratio:
density = mass
volume
For liquids and solids, mass is reported in grams, and volume is reported in milliliter
(mL) or cubic centimeters (cm3), so density is reported in units of grams per milliliter
(g/mL) or grams per cubic centimeter (g/cm3)?these are equivalent since 1 mL?1 cm3.
At the same temperature a substance?s density will be the same regardless of the size of
a sample. For example, at 0?C ice has a density of 0.917 g/cm3, whether you have an ice
cube or a glacier. Because density is a unique characteristic of pure substances, it can be
used to identify an unknown. For example, a metal object that is silver in color may be
composed of aluminum, platinum, or silver. By measuring the density, a chemist can
make a tentative conclusion about the identity of the metal because the three elements
have dramatically different densities, as seen in Table 1 below.
Table I: Densities of common substances at 25?C and 1 atm
substance
helium
air
carbon dioxide
ethanol
water, liquid
ice (at 0?C)
blood
aluminum
titanium
density (g/mL)
0.000179
0.0012
0.0018
0.7855
0.997
0.917
1.027
2.70
4.51
substance
zinc
steel
iron
copper
brass
silver
lead
mercury
gold
density (g/mL)
7.13
7.75-8.05
7.86
8.96
8.69
10.49
11.4
13.5
19.3
The density of a mixture can be approximated as the weighted average of the densities of its
components. For example, sterling silver is an alloy of silver with another metal. If you were
trying to determine if a sterling silver charm is a mixture of silver and lead or silver and copper,
CHEM 121 Lab Manual F2019
page 25
you could measure the density of the sterling silver charm. If the density was determined to be
10.2 g/mL, then you could conclude the charm must be a mixture of silver and copper. You
could also conclude it contains much more silver than copper since the density of the charm is
much closer to silver?s density.
In this lab, you will use density to investigate the composition of a US penny, which has
changed over time. It would be difficult to calculate the volume of an individual coin by
measuring its dimensions (i.e., the diameter and the thickness of the coin), so we will use the
?volume by displacement? method to measure the volume of several coins at once. You will
determine the densities of US pennies minted before 1982 and after 1982, respectively. Then
you will draw conclusions regarding their composition based on the densities determined.
For the second and third parts of the experiment, you will estimate the density of an unknown
liquid and a common gas. Working carefully and precisely on the measurements will be
essential, as part of your grade will be based on your accuracy. All the techniques in the
experiment were learned in Experiment 1, so review the Laboratory Techniques section on using
specific equipment from that experiment if necessary.
Procedure
Note: Always include units in every measurement you record. Calculated values should be
rounded to the appropriate number of significant figures or decimal places.
Work in pairs for parts A and C.
Part A. Density and Composition of Pennies
1. There should be a set of pennies from each time period at each lab bench.
2. Make sure the pennies are dry. Count out between 12 and 15 pennies dated before 1982,
and record the number of pennies used. Using an analytical balance tare a plastic weigh
boat or weighing paper, then place the pennies on the boat or paper, and record their
mass for Trial 1. Use a 50-mL graduated cylinder to estimate the total volume of these
pennies using the ?volume by displacement? method. Be careful not to let any water
from the cylinder splash out when you place the pennies in the water. For Trial 2, use 35 fewer pennies than in Trial 1, and repeat the same procedure, recording all data for
Trial 2. Calculate the density for each trial. The densities for the two trials should agree
within ?0.5 g/mL. If they do not, repeat the trials until you achieve the required
precision. If you repeat the trials more than twice and cannot get the required precision,
consult your instructor. When you finish, dry all the pennies and return them to the
instructor.
3. Obtain a set of post-1982 pennies, and repeat step 1 using the new set of pennies. Again
conduct two trials, and calculate the density for each trial. Carry out additional trials until
two trials agree within ?0.5 g/mL.
4. Calculate the density of pennies from each trial, and then the average density for the pre1982 pennies and for the post-1982 pennies. The two densities should be quite different.
If not, consult your instructor to determine if you need to carry out additional trials.
CHEM 121 Lab Manual F2019
page 26
Part B: Density of an Unknown Liquid
Work individually on the unknown liquid.
Waste Disposal: Dispose of the unknown liquid in the waste container in the hood! Dump
any extra liquid in the waste container, and return the empty vial to the cart. Remember
to peel off the label and put it in your lab notebook or lab report page.
1. Obtain a vial of unknown liquid from the lab cart. Peel off the sticker and put it on your
data sheet. Use an analytical balance to weigh a clean, dry, and empty 10-mL graduated
cylinder, and record its mass. Fill the 10 mL graduated cylinder with the liquid to the
highest marking, and record the volume to the correct number of decimal places. Weigh
the full graduated cylinder with the liquid, and record the mass.
2. Empty the graduated cylinder into a 100 mL beaker at your station. Pour the remaining
unknown liquid into the 10 mL graduated cylinder. Determine the mass and volume of
the liquid, and record both measurements. Empty the second sample in the graduated
cylinder into the same 100 mL beaker, then dispose of the unknown liquid in the Waste
Container in the Waste Hood. Return the empty vial to your instructor.
Part C: Density of a Gas
1. Using an analytical balance measure the mass of a dry 125-mL Erlenmeyer flask with a
rubber stopper, and record the mass. Obtain a small piece of dry ice from your instructor.
CAUTION: Do not handle the dry ice! It can rapidly cause severe frostbite. Your
instructor will use tongs provided to select a small piece of dry ice for you.
2. Allow the piece of dry ice in the flask to sublimate. As carbon dioxide changes to a gas,
it will fill the flask completely, displacing the air, and then gradually come to room
temperature. Wait at least 5 or 10 minutes, until all the solid dry ice has sublimated and
disappeared, before continuing to the next step. Do NOT stopper the flask while solid
dry ice is still present, or the flask may shatter as CO2 builds up! (You may begin
working on the calculations for Part C while you are waiting.)
3. Dry the outside of the flask to remove all moisture from water vapor in the air that may
have condensed on it. Stopper the flask to keep in the remaining CO2 gas. Use an
analytical balance to measure the mass of the flask filled with carbon dioxide and the
rubber stopper, and record the mass.
4. Finally, you must measure the volume of the flask. Fill the flask up to the very top with
water then use the stopper to cap it. Make sure the water fills the flask to the stopper. Use
a 50-mL graduated cylinder to measure the volume of all the water contained. Because
the total volume will far exceed 50 mL, you will need to fill and then empty the
graduated cylinder several times, summing the volume each time to get the total volume.
CHEM 121 Lab Manual F2019
page 27
CHEM 121 Lab Manual F2019
page 28
CHEM 121: Density
Pre-Lab Assignment
Name: ______________________________
1. For Part A, you will measure the volume of a small stack of pennies using volume by displacement (or water
displacement), one of the methods used in Experiment 1. Briefly describe how you will use a 50 mL
graduated cylinder and water to measure the volume of pennies by displacement. (Consider sketching the
graduated cylinder and water before and after the pennies are added.) List the measurements you will make
and how they will be used to calculate the volume of pennies.
2. Explain why you should NOT stopper the flask with dry ice until the dry ice is no longer visible.
3. Consider the data shown below for a rectangular metal sample. A student used an analytical balance and a
centimeter ruler marked to every 0.1 cm, and recorded the following:
Experimental Measurements
Mass of Metal Sample
77.55 g
Length of Metal Sample
4.6 cm
Width of Metal Sample
1.4 cm
Thickness of Metal Sample
1.4 cm
a. Calculate the density of the metal sample, expressing
the final answer with the correct significant figures
and units in the answer.
b. There are two mistakes in his data, based on the type of instruments used to make the measurements.
Identify the mistakes.
4. You will use a 50-mL graduated cylinder in this experiment to measure volume. Given that smallest marking
on this graduated cylinder is 1 mL, circle all of the following sample measurements that are NOT appropriate
or NOT recorded to the correct number of decimal places using a 50-mL graduated cylinder:
44.1 mL
CHEM 121 Lab Manual F2019
30 mL
15 mL
page 29
32.5 mL
15.11 mL
CHEM 121 Lab Manual F2019
page 30
CHEM 121: Density
Name: ______________________________
Partner: _____________________________
A. Density and Composition of Pennies
LAB REPORT
Pre-1982 pennies
Trial 1
Trial 2
Post-1982 pennies
Trial 1
Trial 2
Total mass of pennies
Number of pennies used
Volume of water before
pennies added (Vinitial)
Volume of water after
pennies added (Vfinal)
Volume of pennies
Density
Average Density
B. Density of an Unknown Liquid
Unknown Number: ____________
or place your unknown sticker here.
Trial 1
Trial 2
Mass of grad. cylinder + liquid
Mass of empty grad. cylinder
Mass of liquid
Volume of liquid
Density of liquid
Average Density of liquid
Show the calculation for the average below:
For instructor?s use only
Actual density of liquid: ____________ g/mL
CHEM 121 Lab Manual F2019
page 31
PART C: Density of CO2 Gas
DATA
Mass of dry, empty 125-mL Erlenmeyer flask +
stopper (massempty flask):
Mass of flask after dry ice has sublimated + stopper
(massflask with CO2):
Volume of flask (Vflask), using repeated measurements with a 50-mL graduated cylinder,
expressing each measurement with the correct number of decimal places:
___________ + ___________ + ___________ + ___________ = ______________
CALCULATIONS
1.
Calculate the mass of air present in the empty flask.1 Use the volume of the flask
(Vflask above) and the density of air (1.2 g/L) to calculate mass. Showing all work below.
Mass of air in flask (massair in flask): _______________
2.
Calculate the mass of flask if it were truly empty using the equation below:
masstruly empty flask = massempty flask ? massair in flask = ___________________
3.
Calculate the mass of CO2 in the flask using the equation below:
massCO2 = massflask with CO2 ? masstruly empty flask = ___________________
4. Finally, calculate the density (in g/L) of CO2 gas, showing all work below:
Density of CO2 gas: ________________________
1
Keep in mind that when you weigh an ?empty? flask, it is not really empty; the flask is filled with air. The mass of this air is generally
insignificant when compared to the masses of solids and liquids, but it is significant compared to the mass of other gases, like CO2. When
the CO2 sublimates, it displaces the air. To calculate the correct mass for CO2, the mass of the air must be subtracted from the mass of the
?empty flask?.
CHEM 121 Lab Manual F2019
page 32
CHEM 121: Density
Post-Laboratory Questions
1. Your results from Part A should have demonstrated that the pennies from these two eras have
different compositions. One type of penny is mostly copper (95%) with a thin layer of zinc (5%)
while the other penny is mostly zinc (97.5%) with a thin outer layer of copper (2.5%).2 If the density
of copper is 8.96 g/mL and the density of zinc is 7.13 g/mL, determine which penny is which based
on your results. (Circle one for each statement below.)
The _____ penny is mostly copper (95%) with a thin layer of zinc (5%).
pre-1982
post-1982
The _____ penny is mostly zinc (97.5%) with a thin layer of copper (2.5%). pre-1982
post-1982
Write in complete sentences to support your choice based on your experimental data and results.
2. Ethanol (or ethyl alcohol) is the specific type of alcohol present in alcoholic beverages like beer,
wine, vodka, etc. When ethanol is added to water, the two liquids mix completely with one another.
At room temperature, ethanol?s density is 0.789 g/mL while water?s density is 0.997 g/mL. A
student is given an unknown alcoholic beverage in a beaker to analyze at room temperature, so the
student puts a piece of ice in the unknown liquid and observes that the ice drops below the surface
and floats in the middle of the liquid. Based on this observation above and given ice?s density is
0.917 g/mL, what can you conclude about the density and composition of the liquid?
The liquid is __________. (Circle one)
pure ethanol
pure water
a mixture of the two
If it is a mixture, use density to determine if it contains more ethanol or more water, then write in
complete sentences to explain your conclusions. (Calculations are not necessary for this problem.)
2
https://www.usmint.gov/learn/history/coin-production
CHEM 121 Lab Manual F2019
page 33
3. In your calculation of the density of the unknown liquid (in Part B), which measurement limited the
number of significant figures in the reported density: volume or mass? Suppose a student wanted to
determine the density with more precision. He decided to retake the mass of the liquid on the
analytical balance because he feels this will provide more precision for the experiment. Explain
whether or not this an effective approach.
4. In Part C of the experiment, you determined the density of CO2 gas.
a. How does the density of CO2 compare to the density of the pennies and the unknown liquid and
other solids and liquids included in Table I on page 25?
a. What can you conclude about the density of gases compared to the densities of liquids and
solids? Based on the arrangement and number of particles in solids, liquids, and gases, explain
why the densities of gases are so different from the densities of solids and liquids.
5. A substance of higher density will sink in a substance of lower density: For example, you may know
that oil floats on top of water because it has a lower density. For each of the following pairs,
determine whether the object will sink or float. Refer to Table I on p. 27 and your experimental
results to determine each.
a. A lead brick placed in a bucket of liquid mercury will _________.
sink
float
b. A piece of ice placed in your unknown liquid from Part B will _________.
sink
float
c. A balloon filled with air in a chamber filled with CO2 will _________.
sink
float
CHEM 121 Lab Manual F2019
page 34

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