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Mass, Volume, and Density
2
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Text reference:
Sections 2.7-2.10
Background
An old riddle asks "Which is heavier, a pound of feathers or a pound of
lead?" The question is nonsensical, of course, since a pound of feathers
and a pound of lead both weigh the same, one pound. Nevertheless,
there is clearly something different about a small lead brick and a large
bag of feathers, even though they weigh the same. The key to answering
the riddle is understanding the relationship that exists between a substance's mass and the volume it occupies. This relationship is expressed
by the physical property called density. Density is defined as the ratio of a
substance's mass to the volume it occupies.
Density= mass of substance (g) /volume of substance (mL)
In this experiment, you will measure the mass and volume of several
unknown materials. You will then use your data to explore the relationship between the mass and volume of the materials and to calculate their
density.
After performing this lab, if someone asks you the riddle about
feathers and lead, you can explain to them the difference between weight
and density.
Goals
• Measure the mass and volume of several samples of matter, using the
centigram balance and the method of water displacement.
• Infer whether there is a constant relationship be een
ume for each unknown substance.
• Compute the density of a solid object from its mass and volume.
Procedure
1. Determine the mass of two different unknown metal samples to the nearest
0.01 grams using a balance. Record masses in Table 2.1.
2. Find the volume of each metal sample by water displacement. Fill a
25-mL graduated cylinder about half-full with water (make sure it is enough
water to completely submerge your metal when the metal is added to the
cylinder), measure the volume, and record as "volume of water alone" in
_Table 2.1.
3. Tilt the graduated cylinder and carefully slide metal A down the side. Make
sure the metal sample is completely submerged in the water. Measure the
volume and record the measurement as "volume of water + metal" in Table
2.1.
4. Repeat steps 2 and 3, using the other metal sample B. Dry both samples
and return them to your tray.
5. Determine the volume of just the metal (mL) for sample A and B and record
the data in your data table
6. Determine the density of each metal (Hint: you are using information from
your data table to determine this…look at units!!)
7. Compare your density values to the reference sheet provided by your teacher
and identify metal A and B.
Data Record
Table 2.1: Individual Data and Calculations
Metal A
Metal B
mass (g)
Volume of water alone (mL)
Volume of water + metal (mL)
Volume of metal (mL)
Density (g/mL)
Metal Identity (check data
reference tables)
Lab Pair
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Table 2.2: Class Data: Mass (g) and Volume (mL) of Metal A
Mass (g)
Volume (mL)
Density (g/mL)
Data Analysis (Hand graph)
Note, one page of graph paper is required for your laboratory re port
1._ Using- the class data, plot a graph of mass versus
volume. Represent the plotted points for each metal with
a different symbol. Draw a "best fit" straight line
through each group of plotted points.
'
.
2. Determine the slope of the line on -your graph. Record the slope of the
line and the line equation on your graph. Hint: the general equation
for a line is y = mx + b where m is the value for the slope and b is
the value for the y-intercept. Pay special attention to the units of the
slope.
Data Analysis (Graph in Google Docs)
1. Use a Chromebook provided by your teacher
2. Your teacher will provide instructions for graphing in Google Docs
3. Write down the line equation for your graph in your lab notebook
Conclusions
1. What does the slope of the line for metal A represent? Hint: Look
back at your hand graph line equation and your Google Docs line equation.
2. . Looking at your ·graph, what does this experiment demonstrate
about the density of a substance in reference to class data (Hint: look at
the mass (g) and volume (mL) of metal A for different groups)
3. Calculate the percent error i n t h e d e n s i t y c a l c u l a t i o n for
your individual metal A density sample using the following equation.
Your teacher will provide you with an accepted value for the density of
your metal.
Percent error = | accepted value – experimental value |
accepted value
x 100
4. Calculate the percent error in the value of density obtained from the
slopes of the line in your graph based on the class data.
5. Look back at the percent errors calculated in steps 3and 4. Generally,
the slope of the line will give a more accurate value for density than a
single sample. Explain why this is usually true.
6. Can you identify a substance if you know its density? Explain -your
answer. After referring to the density reference tables, what were both
metals used in this experiment?
7. Do you think that determining the volumes of your metal samples by
measuring their dimensions and calculating the cm3 would be more or
less accurate than determining these volumes by water displacement?
Explain. Would measuring the dimensions of a solid always be possible?
Explain.