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BONUS LAB: Density of Unknowns
You will need to allow approximately 2 hours for completion of this lab, so if you are interested
in earning these bonus points, the lab can be done on the following mornings before school,
beginning at 7a.m. Your final paper is due the beginning of your class period on the date listed
below. REMEMBER, late work for bonus assignments will not be accepted!
Wednesday/Thursday, September 26th/27th (due Oct 1st)
Tuesday/Wednesday, October 3rd/4th (due Oct 8th)
Introduction:
Density is the mass of a substance per unit volume. The density of any substance is a ratio and
may be calculated by dividing the mass of the sample by its volume.
mass
density =
volume
The most common units of measurement for density that you will encounter in this course are g/mL
and g/cm3.
When measured at the same temperature and pressure, all samples of a particular substance
have the same density regardless of the quantity or shape of the sample. Thus, density is a
characteristic property of matter that is often used by chemists to identify a substance.
In this investigation, you will first determine the density of distilled water by finding the
density of three separate samples and calculating an average. You will then repeat this procedure for
an unknown liquid. Finally, you will compare your measured values with the accepted values for the
densities of these liquids.
Prelab Questions: These must be completely answered BEFORE you are allowed to begin the
lab.
1. What are the two most common units for density?
2. Why is the precision of the laboratory balance (triple beam balance) that you use in this
investigation important? What effect would a less precise balance have on your results?
3. What is the advantage of doing three trials of measurements instead of just one when
determining the density of each liquid?
4. Why should you avoid skin contact with the unknown solution?
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Materials:
Safety goggles
triple beam balance 10 mL graduated cylinder
distilled water
100 mL graduated cylinder unknown liquid
2 unknown metal samples
Procedure:
Determining the Density of Water
1) Mass a 10-mL graduated cylinder to the nearest 0.01 g and record data in Table 1.
2) Fill the graduated cylinder with 4.0-5.0 mL of distilled water and mass it again. Record in Table 1.
3) Determine the volume of the water from step 2 by reading the value on the graduated cylinder.
Read the bottom of the meniscus at eye level to the nearest 0.1 mL. Record data in Table 1.
4) Repeat steps #1-3 by increasing the volumes of water each trial.
5) Empty the graduated cylinder, dry it.
Determining the Density of an Unknown Liquid
6) Write the # of your unknown liquid in Table 2.
7) Record the mass of the 10-mL graduated cylinder in Table 2. (be sure it is dry before you
proceed)
8) Repeat steps #2-5 with your unknown liquid.
9) Dispose of your unknown liquid according to your teacher’s instructions.
Data:
TABLE 1: Distilled Water
Trial 1
Trial 2
Trial 3
Average
Density
Mass of graduated cylinder and water (g)
Mass of empty graduated cylinder (g)
Mass of water ONLY (g)
Volume of water (mL)
Density of water (g/mL)
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TABLE 2: Unknown # ________
Trial 1
Trial 2
Trial 3
Average
Density
Mass of graduated cylinder and unknown (g)
Mass of empty graduated cylinder (g)
Mass of unknown (g)
Volume of unknown (mL)
Density of unknown (g/mL)
Calculations:
1) Calculate the mass, volume and density of the water and unknown and write in the data table.
2) Determine the average density of water: (add up densities and divide by 3 trials)
3) Determine the average density of the unknown liquid: (add up densities and divide by 3 trials)
TABLE 3: Accepted Values for Density of Various Liquids
LIQUID
Ethanol
Isopropyl alcohol
Methanol
Mineral oil
Hydrogen Peroxide
Glycerin
DENSITY (g/mL) at 20 C
0.789
0.786
0.791
0.830
1.110
1.261
Determining the Density of 2 Unknown metals:
1. Obtain 2 metals from your teacher and record their numbers in the appropriate place in the Table 4.
2. One at a time, place the metal samples onto the balance to determine the mass to the nearest 0.01g.
Record in Table 4.
3. Add water to the 100 mL graduated cylinder and record this volume as VOL of water in table 4.
4. Tilt your graduated cylinder and carefully slide your metal sample into the water. Read the new
volume level and record this volume as VOL of water + metal.
5. Repeat steps 2 – 4 for a second trial of the same metal.
6. Repeat steps 2 – 5 for the other unknown metal.
7. Be sure to dry off all metal samples and return them to your teacher.
8. Clean you station and leave it just as you found it.
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TABLE 4
Mass (to nearest 0.01g)
1st Unknown #_____ 2nd Unknown # ______
VOL of water (to nearest
0.1mL)
VOL of water + metal (to
nearest 0.1mL)
Volume of unknown (mL)
Density of unknowns (g/mL)
Average density of unknown
(g/mL) – (add densities and
divide by 2)
TABLE 5: Densities of various common metals
METAL
DENSITY (g/mL)
Aluminum
2.71 g/mL
Tin
7.28 g/mL
Iron
7.85 g/mL
Brass
8.60 g/mL
Copper
8.92 g/mL
Lead
11.34 g/mL
Critical Thinking Questions:
1. Based on your data, which of the liquids in Table 3 most likely is your unknown liquid?
2. If your unknown liquid had a density of 0.79 g/mL and you knew that it was one of the
three substances listed in Table 3 having a density close to your value, how would you go
about determining which liquid was your unknown?
3. When stating density of a liquid, why is it necessary to state the temperature (HINT: What
do you think happens to liquids if they change temperatures)?
4. According to Table 5, which metals are most likely your metals?
I predict unknown metal #1 to be __________________
I predict unknown metal #2 to be __________________
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5. Suppose a student read the TOP of the meniscus (instead of the bottom). Would this cause
the calculated value for density to come out too high or too low? Explain why you think so.
6. Suppose the student in question #5 forgot to subtract off the mass of the graduated cylinder
while finding the mass of the liquid. Would this cause the calculated value for density to come
out too high or too low? Explain why you think so.
7. Suppose that a student splashed some water out when she dropped her metal sample into the
cylinder. Would this cause her density calculation to be too high or too low? Again, explain
why you think so.
8. Let’s say a sample of your unknown metal turns out to be 3.0 g/mL. If it was cut exactly in
half, what would the density be of each of the 2 new halves?
Why do you think so?
9. Why is the property of density so important?
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