Chemistry 400
Experiment 1 – Basic Techniques
I.
Discussion
In our first experiment, the most basic techniques of measurement will be introduced: using
a balance, measuring a volume of liquid, and measuring temperature. These three techniques
will be combined to determine the accuracy and precision of three types of glassware (beaker,
buret, and volumetric pipet) for measuring the density of water. The purpose of this lab is to
determine the differences and similarities in accuracy and precision for these three types of
glassware.
Figure 1 – Several types of glassware used to measure volume. a = beaker,
b = buret, c = volumetric pipet, d = graduated cylinder, e = volumetric flask
The analytical balances in the Chemistry 400 balance room measure the mass of an object
to four decimal places: an object with a mass of between 1 and 9 grams will have five significant
figures, and an object with a mass of 10 or more grams will have six significant figures. This will
be more significant figures than the volume measurements. Therefore, the mass measurements
will not limit the determinations of accuracy and precision for the glassware.
Figure 2 – An analytical balance
Accuracy is a measure of how close a value is to the true value. For example, if you are
trying to measure out 20.00 mL of water and you actually measure out 18.05 mL using one type
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of glassware and 19.99 mL using a second type of glassware, then the second type of
glassware would be considered more accurate at measuring out 20.00 mL of water. One
measure of the accuracy is the % error, defined in Appendix A.
Precision is a measure of the reproducibility of multiple measurements. For example, if you
measure out 20.00 mL using two different types of glassware you might get the following
results:
Glassware type #1 (mL) Glassware type #2 (mL)
19.57
19.55
19.60
19.32
19.58
19.88
Because the results for glassware type #1 are closer together, they are more precise. Note that
these two sets of data have the same average, so they would have the same accuracy but
different levels of precision. One measure of the precision of a group of measurements is the
standard deviation, defined in Appendix A.
For measuring volume using a pipet, there is a "fill line" inscribed on the neck of the pipet.
The exact position of the fill line varies for different pipets. However, the fill line is marked at
exactly 20.00 mL when the bottom of the meniscus is positioned exactly on the line. To record
the volume of a 20.00 mL pipet in your notebook, record "20.00 mL" with four significant figures.
Measuring a volume using a buret requires taking the difference between a starting and
ending volume. The simplest starting volume is often 0.00 mL.
When using a balance, the significant figures are displayed for you. However, a buret and a
thermometer (at least the thermometer used for this lab) require "reading" a graduated scale
correctly. When reading a graduated scale, always record the measurement to one more
decimal place than the graduations on the scale. In this way, the last decimal place is an
estimate (i.e., it is not known exactly). Keep this concept in the back of your mind: the way that
measurements are recorded using significant figures is that all of the KNOWN digits are
recorded, and then one more is recorded for which there is some (but not complete) uncertainty.
To read a buret: a buret generally has graduations every 0.2 mL: record the volume to the
nearest 0.01 mL. A thermometer has graduations every 1°C: record the temperature to the
nearest 0.1°C.
II.
Objectives
1. To learn the techniques of measuring mass, volume and temperature.
2. To determine the accuracy and precision of three different types of glassware for
measuring the density of water.
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Table 1. The density of water as a function of temperature.
III.
Procedure – You will work on this lab independently (no lab partners).
A. Using a 20.00 mL Pipet to Measure Volume
1. Draw about 400 mL of deionized (DI) water into a clean beaker and let it EQUILIBRATE
TO ROOM TEMPERATURE. To do this means to allow a solution to stop changing
temperature when it reaches room temperature. Once the solution reaches room
temperature, it will not change temperature any more. It will have equilibrated. This will
take at least 15 minutes before starting to take temperature measurements, so set this
water aside
Note: There is only one DI water tap in the lab room; make sure you use the correct tap.
2. RECORD THE MASS of a clean 125 mL Erlenmeyer flask. To do this use the analytical
balances located in LH208 (preferably the same one for all measurements). Make sure
that the outside of the flask is dry and that the balance pan is clean and dry. With
nothing on the balance and the sliding doors of the balance all closed, press the "tare"
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button to zero the balance. Then place the flask on the balance pan and close the doors.
ALWAYS record all the digits on the balance’s display. Record the mass in Table 1.
3. RECORD THE TEMPERATURE OF THE WATER that was set aside in step 1 and the
room. To do this hold the thermometer in the middle of the solution until the temperature
stops changing, which may take several minutes. Practically speaking, you may use a
buret clamp (generally found in the large cabinets between the lockers) to hold the
thermometer at the right position–in the middle of the solution. Because the markings on
the thermometers are in 1°C, the temperature must be recorded to the nearest 0.1°C.
The two temperatures (water and room) should agree before you continue.
Note: If the thermometer is touching the bottom of the beaker, then the thermometer is
measuring the temperature of the water and the lab bench.
4. CONDITION THE PIPET WITH DI WATER. To do this squirt DI water from a squirt
bottle down the inside of the pipet (from the top and without a pipet bulb) while slowly
(and carefully) rotating the pipet such that the entire inside of the pipet gets rinsed with
DI water. During the conditioning process, the water should "cascade" down the inside of
the pipet with out any evidence of dirt or without leaving droplets stuck on the insides.
The presence of such drops indicates that your pipet is dirty.
5. DELIVER 20.00 ML of the equilibrated water USING THE PIPET into the Erlenmeyer
flask. Fill the pipet such that the bottom of the meniscus is exactly touching the line on
the neck of the pipet. Your pipet is a transfer pipet that is calibrated “to deliver” (TD)
rather than “to contain” (TC). The last drop of liquid should not drain out of the tip of a TD
pipet in normal use. To ensure complete transfer of the correct amount of fluid from a TD
pipet, make sure to touch the tip of the buret against the inside of the flask 2-3 times
once the pipet has finished draining. If you have any questions about proper pipetting
technique, your instructor can observe the first time you pipette to ensure that your
technique is good.
6. Record the mass of the flask and the water.
7. Repeat steps 5 and 6 at least two additional times without emptying out your flask
between trials. This means that you will have 20.00 mL in the flask after the first trial,
40.00 mL after the second trial and then 60.00 mL after the third trial.
8. If you are using a pipet to deliver substances other than water, be sure to rinse the pipet
with tap water followed by a final rinse with DI water.
B. Using a Buret to Measure Volume
1. Discard the water in your Erlenmeyer flask and re-measure the mass of the flask. The
inside of the flask need not be completely dry because any water left in it is from the
previous procedure and is at the same temperature as the new water you will be adding.
2. CONDITION THE BURET WITH DI WATER. The buret should be stored with DI water
in it. Just in case it is not (good scientific practice), you will condition the buret with DI
water. First, drain the buret of the water. Then close the stopcock and squirt 3-5 mL of DI
water into the buret making sure to run the water down all of the inside of the buret.
Open the stopcock and allow the DI water to go through the tip. Repeat with 2 more 3-5
mL volumes of DI water. Similarly to the pipet, the water should "cascade" down the
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inside of the pipet with out any evidence of dirt or without leaving droplets stuck on the
insides.
3. Record the temperature of the water.
4. DELIVER AN EXACT VOLUME of the equilibrated water FROM THE BURET. Use a
buret to accurately measure out between 23 and 25 mL of room temperature DI water
from part A into the flask. Your eyes should be at the level of the liquid surface in the
buret. Read the value at the bottom of the meniscus. You should read the buret to the
closest 0.01 mL (e.g., 3.14 mL). See the illustration below. In your laboratory notebook
in Table 2, record your initial buret reading and your final buret reading. The volume of
water delivered by the buret is the difference between the final and initial buret reading.
Oftentimes a simple initial buret reading is 0.00 mL.
5. Record the mass of the flask and the water.
6. Repeat steps 3 and 4 at least two additional times without emptying out your flask
between trials. This means that you will have about 24 mL in the flask to start with but
then you will have about 48 mL and then about 72 mL as a final volume.
7. If you are using a buret to deliver substances other than water, rinse it with plenty of tap
water followed by a DI water rinse. Fill the buret with DI water before storage. If you are
using acids or bases, you will need to be sure that the conditions inside the buret are
neutral (pH =7) before you put it back on the rack.
C. Using a Beaker to Measure Volume
1. Choose a clean and dry 100 or 150 mL beaker that has graduations (or marks) for each
10 mL or 20 mL (one should be in your locker). Record the mass of the beaker in Table
3.
2. Record the temperature of the water.
3. Measure out 5.0x101 mL of the room temperature DI water used in parts A and B.
4. Record the mass of the beaker and the water.
5. Empty out your beaker and carefully measure out another 5.0x101 mL of your room
temperature water. There is no need to reweigh the empty beaker.
6. Record the mass of the beaker and water.
7. Repeat steps 4 and 5 at least once more.
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IV.
Calculations
A. Calculations for the Pipet and Buret
Note: it is important statistically that we treat each trial independently. Therefore, you want
to determine the density of each 20.00 mL trial for the buret independently of each other
trial. This means that none of the calculations for the pipet will use a volume of more than
20.00 mL and none of the calculations for the buret will use a volume of more than 25.00
mL. If you have any questions about what this means, please ask your instructor.
1. Calculate the mass of water delivered by your pipet for each trial independent of each
other trial. Remember, you did NOT empty the Erlenmeyer flask for trials 2 and 3.
Therefore, to get the mass of water for Trial 2, subtract the "Mass of Flask and Water"
for Trial 1 from the "Mass of Flask and Water" for Trial 2. This calculation is similar for
Trial 3.
2. Use your mass of water and the volume of the volumetric pipet to calculate the density of
water for each trial. Calculate the average density and the standard deviation.
3. Determine the percent error with respect to the average density of water at the
temperature of your water. The temperature may be different for each set of calculations.
4. Repeat the calculations for the buret.
B. Calculations for the Beaker
1. Use your mass of water and the volume of the water measured by your beaker to
calculate the density of water for each trial. Calculate the average density and standard
deviation.
2. Determine the percent error with respect to the average density of water at the
temperature of your water.
Results & Discussion - Follow the requested format for this section (See page 11).
The following will serve as a guide for you.
Results Table: Summarize your results in a table (Table 4) that lists each type of
glassware with its density value, standard deviation and percent error. The density and
standard deviation should be listed as follows (Be sure you know how to report standard
deviation to the correct number of significant figures):
density ± standard deviation
Trends: 1) Rank the three pieces of glassware from least to most accurate. Discuss
your results with your classmates. Did your results generally agree or disagree with their
results?
2) Rank the three pieces of glassware from least to most precise. Discuss your
results with your classmates. Did your results generally agree or disagree with their
results?
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Error: State possible sources of error here. Explain how to correct these sources of error
in your conclusion… not in your results section.
V.
Conclusion
Follow the requested format for this section (See page 12). Be concise but thorough
enough that whoever reads your conclusion knows how you did your experiment and
the outcome WITH NUMERICAL VALUES. The reader should also know how to
improve the experiment if they should try to reproduce your results.
VI.
Questions
Make sure you have already answered the Trends
and Sources of Error before answering the
questions.
1. Suppose you are required to measure out
exactly 20.00 ± 0.02mL of a liquid. Which
piece of glassware would you use?
2. In mL, what is the volume of liquid in the graduated
cylinder pictured on the right (to the correct number
of significant figures)?
3. For the graduated cylinder pictured on the right,
where would you expect it to rank as far as accuracy
and precision in comparison with the other three
pieces of glassware. Why?
4. You accidentally blow out all of the water (without
noticing) from your "To Deliver" pipet. How would
this affect your calculated density?
5. In Part C, why did you have to initially weigh a dry beaker, but for the subsequent
measurements, the beaker did not have to be dried?
6. We could have performed this experiment with cooking oil instead, which has a lower
density than water. If you measured out equal volumes of vegetable oil and water,
which would have the greater mass?
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Experiment 1 – Basic Techniques
DATA TABLES
Table 1: Part A – Using a 20.00 mL Pipet to Measure Volume
T = ___________
Trial 1
Mass of 125 mL
Erlenmeyer Flask
Trial 2
Leave Blank. See
procedure and …
Trial 3
… calculations for
details.
Trial 4 (if needed)
Leave Blank.
Trial 3
… calculations for
details.
Trial 4 (if needed)
Leave Blank.
Mass of Flask and
Water
Mass of Water
Volume of Water
Calculated
Density of Water
Table 2: Part B – Using a Buret to Measure Volume
T = ___________
Trial 1
Mass of 125 mL
Erlenmeyer Flask
Trial 2
Leave Blank. See
procedure and …
Mass of Flask and
Water
Mass of Water
Initial Buret
Reading
Final Buret Reading
Volume of Water
Calculated
Density of Water
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Table 3: Part C – Using a Beaker to Measure Volume
T = ___________
Volume of water in beaker (with proper sig figs) = ____________
Trial 1
Mass of Beaker
Trial 2
There is no need
to reweigh the
empty beaker …
Trial 3
… because it's not
empty (dry)
anymore.
Trial 4 (if needed)
Leave blank.
Mass of Beaker and
Water
Mass of Water
Calculated
Density of Water
Table 4: Results Table
Volume Measured with
20.00 mL Pipet
Volume Measured with
Buret
Temperature
Correct Density of
Water at this Temp.
Average Density of
Water
Standard Deviation
Percent Error
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Volume Measured with
Beaker