Laboratory
1
Scientific Measurements
Objectives
•
Develop expertise with the analytical balance and commonly used laboratory glassware.
•
Understand the uses and limitations of each so that the appropriate lab
equipment can be chosen for future experiments.
•
Determine the precision and accuracy of your results.
Introduction
Suppose that you have a glass measuring cup. The volume of water present inside
the measuring cup is indicated by lines marked on the outside of the glass. You
read the initial volume using these markings, pour some of the water into a mixing
bowl, and then read the final volume. With these two values you can calculate the
volume of water that you transferred to the bowl. Would this measured volume
of water that you transferred from the glass to the bowl then be equivalent to the
actual amount of water transferred?
This may sound like a silly question. You may think, “Of course it is the actual
amount, I just measured it!” But what if the lines that are marked on the glass are
not very accurate? What if you were sloppy when reading the volume, or when
pouring the water? It is quite possible the measured volume and the actual volume
are different, and the amount of water added to the bowl is not what you initially
thought it was.
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Laboratory 1
The proper use of laboratory equipment is an important part of General Chemistry— so important that it can have a large effect on your laboratory results (and your grade). Therefore,
in this experiment you will learn to use different laboratory glassware to properly transfer
a liquid from one container to another (a very common laboratory task). Comparing the
actual amount transferred with the amount you intended to transfer will provide insights
into the accuracy of the different glassware, the proper use of the glassware, and your skills
using the glassware.
Discussion
Pure water has a well-established density (mass/volume ratio) that depends on its temperature; the accepted value for the density being 1.000 g/cm3 at 4 °C. For the range of
temperatures that are typically associated with “room temperature” (about 20 °C to 30 °C)
the density of pure water is given by the following equation, determined experimentally:
density f
g
cm3
p = f –0.00030
g
cC cm3
# temperature (cC) p + 1.0042
g
cm3
Suppose a chemist is using laboratory glassware to transfer some amount of water to a
beaker. The chemist measures a volume of 25.0 mL (where mL, or milliliter, is a unit of
volume; 1 mL = 1 cm3). How can you determine if 25.0 mL of water or a somewhat different volume was actually transferred by the chemist? In knowing the temperature, and
hence the density, you can determine the actual volume transferred if you know its mass.1
With this information, you can compare the volume of water measured using the laboratory
glassware to the actual amount transferred based on its mass and density. You can then calculate the percent error associated with these measurements using the following equation:
percent error (%) =
actual volume (mL) – measured volume (mL)
actual volume (mL)
# 100%
Remember, the actual volume is the volume calculated using the mass and density and the
measured volume is that measured by the chemist using the markings on the glassware.
For additional resources to help you determine the correct number of significant figures to
report for your calculations, see Appendix F: Treatment of Numerical Data.
Reading Scale Values
50
Meniscus
Eye level
30
20
2
1 ©Hayden-McNeil, LLC
40
The volume of a liquid is measured in graduated (marked)
glassware by recording the position of the bottom of the meniscus, which is the curved surface of a liquid present in a
tube or cylinder. As shown in the figure to the left, it is important to align your eye perpendicular to both the glassware
and the bottom of the meniscus in order to obtain an accurate
reading. Glassware like graduated cylinders, pipets, and burets are read in this manner.
When reading a scale value, the number you record must include all the digits known exactly plus one that is estimated.
This assumes the mass measurement is at least as accurate as the volume measurements, which, in this
case, is an appropriate assumption given the analytical balances provided in the lab.
Scientific Measurements
In this example, the bottom of the meniscus is clearly greater than 36.0 and less than 37.0,
but in the absence of markings between those two values the tenths place must be estimated; therefore, a reasonable estimate is 36.5 mL.
In this lab, you will transfer water using two types of glassware: the buret and pipet. You
will work with your group to compile data, but you will do your own individual transfers
with each piece of glassware.
Spot check: How many decimal places can you read from a buret? A pipet?2
Materials
EquipmentCommon Equipment
glass thermometer
2 100-mL beakers
25.00-mL buret buret clamp
5.00-mL pipet
10.00-mL pipet
analytical balance
Chemicals
distilled water, H2O
Procedure
1. Set up the data tables in your laboratory notebook as shown on the Report Sheets (pgs.
7–8). Record all information directly into your notebook.
2. Add some distilled water to a beaker and measure the temperature of the water. Calculate the density of the water at this temperature using the equation given on pg. 2.
3. Measure the mass of a second, empty beaker. Using the buret, transfer a measured volume of distilled water into the second beaker. The amount of water that you transfer is
at your discretion. Measure the mass of the beaker and the water.
Spot Check: How does the volume of water transferred affect the number of significant figures in your final percent error?
4. Calculate the mass of the water that has been transferred to the second beaker. Based
on the density of the water that you calculated in step 2, calculate the actual volume of
water that has been transferred to the second beaker.
5. Calculate the percent error using the equation on pg. 2 and record it in Table 1.1.
6. Use Table 1.3 to record the percent error data from all group members (including your
trials). Make sure to record the chemist initials to identify group members.
2 Spot Check questions are not graded. These are to help guide you through the lab—they can be discussed
with your group or your TA.
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Laboratory 1
Spot Check: Which chemist performed the best transfer with the buret? Was the
percent error similar for everyone in the group?
7. Have your TA check your group’s work before moving on to the pipet.
8. Repeat steps 3–6 twice using a pipet. You will use the 10.00-mL pipet for the one trial,
and the 5.00-mL pipet for the other trial. It is OK if group members use the pipets in a
different order. Use Tables 1.2 and 1.4 for the pipet data.
Spot Check: Which chemist performed the best transfer with the pipet?
9. Have your TA check your group’s work before cleaning up your space.
Q: What is a Report Sheet?
All data taken in lab is written in your notebook. Report Sheets are used to
organize data for your report after lab. They serve the purpose of tables, charts,
and figures that contain data and observations. Report Sheets are labeled and
located in your lab manual at the end of each experimental procedure. A Report Sheet MUST:
4
•
Include all data to the correct number of significant figures.
•
Be completed after lab.
•
Be turned in on Carmen with any additional report requirements.
Scientific Measurements
Lab Reports
Each student in the group is responsible for writing an individual lab report. You may have
worked with classmates to collect and discuss data during lab, but your analysis and presentation of findings in the report must be your own work. A description of the parts of a
lab report is included in the table below; for additional help, see Appendix B.
Item
Description
Title Page
A cover page with information about the report’s author and
contents; this page should include the course number, your name,
your TA’s name, the date the experiment was finished, and the
date the report was submitted.
Purpose
Identifies the goal of the experiment; 1–2 sentences (in your own
words) describing the purpose of the lab.
Procedure
Information on how the experiment was carried out. Reference
the lab manual with an appropriate citation. Note any procedural
changes you made as you performed the lab.
Data/Report Sheets Specific data tables, or student-generated data tables, are included
in most experiments.
Sample
An example of representative calculations. More information
Calculations
about specific calculations for each experiment can be found on
the rubrics.
Graphs/Figures
Any plots generated during or after the experiment.
Specific questions may be provided at the end of some
Report Questions
experiments.
Three paragraphs that explain what specific information was deDiscussion
termined in the experiment that you have completed.
Conclusion
A summary of the entire experiment and the results obtained.
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Laboratory 1
6
Water
Density
(g/cm3)
Empty
Beaker
Initial
Mass (g)
Trial
Pipet
Used
Water
(5.00 mL
Temp (°C)
or 10.00
mL)
Water
Density
(g/cm3)
Table 1.2 Transfer of water with a pipet.
Trial
Water
Temp (°C)
Table 1.1 Transfer of water with a buret.
Empty
Beaker
Initial
Mass (g)
Buret
Initial
Reading
(mL)
Pipet
Volume
(mL)
Buret
Final
Reading
(mL)
Beaker
+ Water
Mass (g)
Beaker
+ Water
Mass (g)
Mass of
Water (g)
Mass of
Water (g)
Actual
Volume
of Water
(mL)
Actual
Volume
of Water
(mL)
% Error
% Error
Laboratory 1 Report Sheet
Scientific Measurements
Name: __________________________________ Date:_______________ TA Name:_____________________
For Tables 1.1 and 1.2, enter your individual data. For Tables 1.3 and 1.4, enter your group
members’ data as well as your individual percent errors. Actual volume is calculated using the
density of water (equation on pg. 2) and its measured mass.
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Laboratory 1 Report Sheet
Table 1.3 Group percent errors
with buret.
Table 1.4 Group percent errors
with pipet.
Chemist Initials
Trial
% Error
Chemist Initials
Trial
% Error
Supporting calculations (density of water, mass of water, measured volume, actual
volume, total volume, and percent error):
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Scientific Measurements
Data Analysis
Scientists commonly summarize and analyze data using tables, figures, and/or calculations.
Now that you’ve collected data, it’s appropriate to work it up, so to speak. Plot the percent
error results for your group using the figures below. Label divisions for the x-axis scale
using numbers that are appropriate for the range of values of error of your group.
EXAMPLE: You should include marks for every point and appropriate scale readings.
Negative
–3.0
X
–2.0
X
–1.0
Positive
X
0.0
1.0
2.0
3.0
Figure 1.1 Percent error for buret.
Negative
Positive
0.0
Figure 1.2 Percent error for pipet.
Negative
Positive
0.0
Prepare a table below that summarizes the range of percent error values and the average
percent error values for the glassware in your notebook.
Table 1.5 Percent error calculations for laboratory glassware.
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Laboratory 1
Report Questions
1. Suppose an experiment called for the transfer of 10.00 mL of a liquid and accuracy and
precision of the volume was important. What glassware would you use? Explain your
reasoning and use supporting data from this experiment.
2. Suppose an experiment called for the transfer of 17.50 mL of a liquid and precision
of the volume was important. What glassware would you use? Explain your reasoning
and use supporting data from this experiment.
3. In this experiment you measured or calculated several properties of matter, including temperature, mass, volume, and density. Which of these are intensive properties
and which are extensive properties? In your answer communicate an understanding of
these terms and use data to support your reasoning.
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Scientific Measurements
Points to Consider
•
Do the experimental results indicate the presence of determinate errors? Of indeterminate errors?
•
Take a look at your buret data. Discuss your accuracy and precision using just your
information. Identify sources of error that could have affected your accuracy and
precision.
•
Take a look at your group’s average percent errors for the pipet. Discuss the group’s
accuracy and precision as a whole.
•
Reflect on how the equipment you used in lab affected the significant figures of your
final calculations.
Use these points to consider to shape your discussion. For full
points, address each point thoroughly and support your conclusions
with data.
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Laboratory 1
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