CHEM& 151
Fall 2009
BASIC EQUIPMENT, MEASUREMENT, DENSITY,
AND GRAPHING
Prelab attached (p. 17-18)
Fill-in - Refer to your Laboratory Procedures handout for how to do your fill-in lab reports.
Name _____________________
Stamp Here
Lecture Instructor
Partner ___________________
_____________
Date ______________________
Have you attached the graphs and the conclusion?
LEARNING OBJECTIVES After completing this experiment, you should feel comfortable with:
•
Familiarize yourself with the identity, location and use of common laboratory equipment.
•
Experimentally determining the densities of solid or liquid objects.
•
Using various types of volume-measuring devices, and understand the difference in precision
between the devices.
•
Performing percent difference calculations and report the values with correct significant figures.
•
Using a graphing program to graphically render data and analyze the graphs to make conclusions
about the data.
TO EARN YOUR FINAL STAMP: The following items must be completed in the lab. You may
complete the entire assignment in lab, this represents the minimum required to earn your final stamp.
 Complete Activity 1, the equipment scavenger hunt.
 Collect the data for Activity 2, and calculate the density of your object. (Complete all of Data
Table 1)
 Collect the data for Activity 3, completing Data Tables 2, 3 and 4.
 Using a graphing program, use a computer to generate the three graphs needed to analyze the
data, and report your three densities in Data Table 5.
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INTRODUCTION
Any comprehensive course in chemistry must include a certain amount of laboratory time. In a
chemistry laboratory, you will learn how to develop proper lab techniques, carefully observe
experimental results, and accurately interpret data to arrive at a desired solution to a chemical problem.
The laboratory also allows you to observe how the chemical principles and theories presented in lecture
apply to real life situations. The development of good laboratory techniques is essential in obtaining
precise, accurate experimental results. It is, therefore, important to develop these skills early in the
quarter.
The measurement of the physical properties of pure substances is a very important technique as a
part of the larger scheme of identifying elements and compounds. In learning to measure densities, you
will also learn about handling significant figures in measurements and calculations, precision in
measurements and in glassware, and how to generate, manipulate and analyze data in graphs.
SIGNIFICANT FIGURES
Precision: A measurement with a greater number of significant figures is more precise than a
measurement with fewer significant figures. (For example, 5.0g is more precise than 5g)
Accuracy: Accurate measurements are correct measurements. A digital watch, for example, might have
a high degree of precision and measure hundredths of seconds, but if it is running ten minutes late, it is
not accurate.
Each time you make a measurement, you must pay attention to the markings (divisions) on the
measuring device to determine how many digits (significant figures) to record in your answer. Correct
use of significant figures is important throughout this remainder of this experiment and all subsequent
experiments.
Significant figures in your measurements:
•
The number of decimal places that you can legitimately record off of an instrument is the precision
of that instrument.
•
Digital instruments (example balance): record every digit given. For example,
if the balance readout looks like this, record 1.70g, not 1.7g.
•
1.70
Manual instruments: always include an estimated digit. This means that you will g
estimate the digit
after the last marking. For example, using the centimeter ruler in Figure 1, the pencil might be
recorded as having a length of 1.87cm. The markings on the ruler are to the tenths place, so an extra
digit in the hundredths place is estimated by you, the reader. In this number, the “7” is the estimated
digit. Another acceptable reading would be 1.86 cm or 1.88cm, but any other number of significant
figures (or number of decimal places) would be incorrect. For example, 1.9cm would be an incorrect
reading.
0
1
2
3
4
5
Figure 1. Pencil image from the Yale University Picture Dictionary.
http://berlin.cls.yale.edu/picturedictionary/pub/word.asp, accessed 2-21-07.
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Significant figures in measurements that someone else has made:
•
All non-zero numbers are significant
4.56cm includes 3 s.f.
•
Zeroes between non-zero numbers are significant
10.77mL includes 4 s.f.
•
Leading zeroes are never significant
0.01077L includes 4 s.f.
•
Trailing zeroes are significant in the presence of a decimal place
123.70g includes 5 s.f.
120.0mL includes 4 s.f.
•
Trailing zeroes in the absence of a decimal place are ambiguous, and are generally assumed to be
non-significant unless more information is available. It is best, in this situation, to write the number
in scientific notation, this ensures no ambiguity regarding the number of significant digits in the
number.
2300g might have 2, 3, or 4 s.f.
To correctly indicate the number of sig figs in this number, put it in scientific notation:
2.300 x 103g includes 4 s.f.
2.30 x 103g includes 3 s.f.
2.3 x 103g includes 2 s.f.
Significant figures in calculations:
•
Multiplication and Division: the result is rounded to the same number of significant figures as the
least precise number in the calculation:
⎛ 0.70g ⎞
5.67 mL × ⎜
⎟ = 3.969g ⇒ round to 4.0g (2s.f.)
⎝ mL ⎠
•
Addition and Subtraction: the result is rounded to the same number of decimal places as the least
precise number in the calculation:
121.0g - 4.34g = 116.66g ⇒ round to 116.7g (round to tenths place): the least precise number is
only known to the tenths place, so the answer can only be reported
to the tenths place.
•
When you perform a series of calculations, round after each different operation. For example, if a
calculation involves both an addition/subtraction and a multiplication/division, perform the addition
and/or subtraction first and get that answer to the correct number of significant figures. Then use the
adjusted value when performing the multiplication and/or division. It is important to remember, that
you can gain or lose significant digits when you add and/or subtract numbers together!
Percent Difference Calculations A calculation with significant figure ‘precautions’!
The percent difference (or percent deviation) is calculated when you want to know how close an
experimentally determined value is to a given or accepted value (such as one looked up in a reputable
handbook or text). The calculation has both a subtraction and a division, so care must be taken to
determine the number of significant figures that remain from the subtraction, which is then carried
through to the division.
Percent Difference =
#2 Basic Equipment
accepted value - experimental value
x 100%
accepted value
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PRECISION VERSUS ACCURACY
When we collect data in the lab, our ultimate goal is to have both precise and accurate results.
Precision describes data that falls into a narrow ‘window.’ We can measure this window in two ways,
depending on how we collect data. If we make the same measurement numerous times, precision is then
defined as the reproducibility of the results. If your answers are grouped together and you can get the
same reading each time, your results are precise. A measuring device that allows you to read many
decimal places can provide increased precision in the measurement. If we make several different
measurements across a range of values and then graph these data to obtain a result, then precision is
assessed as the correlation of that data to a best-fit trendline. The R2 value is a quantity provided by
most graphing programs (such as Excel), and describes how well the data fits a straight line. Very
precise data would have an R2 value of ONE (1.000…). Any fluctuations due to imprecise measurement
would result in an R2 value less than one; therefore, smaller R2 values indicate less precise measurement.
Accuracy is how close your value is to some “true value”. This “true value” may be a value obtained
from a textbook or handbook, or may also be a class average of many experimentally-determined data
points.
Consider the following is a set of data collected by one student of the length of a particular object: 42.56
cm, 42.55 cm, 42.58 cm. The students’ average is 42.56 cm. The overall class average using the same
type of measuring device was 41.72 cm. The student made very precise measurements – their data was
clustered within a 0.03 cm range, but the results were not very accurate when compared to the class
average.
VOLUME AND MASS MEASUREMENTS (Refer to the appropriate sections in the Laboratory
Handbook for information about using measuring devices.)
Discussion of Volume Measurements
Almost all chemical experimentation requires accurate measurement of some physical or chemical
property. In this experiment you will learn to use the graduated cylinder, the graduated pipet, and the
beaker as a means of measuring the volume of a liquid. The precision and accuracy of the three methods
of measurement will be compared using water as the liquid. Water is attracted to glass, so instead of
forming a flat surface, it forms a concave surface (curves upward at the outer edges) so all volume
measurements should be made at the bottom of the curved surface when using glass devices. This
curvature is called the meniscus.
Figure 2 shows the correct eye position to
use when reading the volume. Incorrect
positioning (called parallax) can result in a
volume measurement that is either too large
or too small. The correct reading of this
volume would be 82.0 mL.
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Beakers
Beakers are designed to give an approximate volume measurement. They come in a
variety of sizes ranging from those which will hold only a few mL to others which
hold many liters. You will use many of these sizes in lab. The precision and
accuracy of the measurement depends on the size of the beaker, but the
measurement is never more precise than a whole number value (i.e., 40 or 45 mL,
never 45.3 mL). Beakers have a large width as compared to height (see Figure 3)
which adds to the difficulty in precisely and accurately reading the volume
Figure 3: a
measurement. Beaker volumes are always whole number readings.
typical beaker
Graduated Cylinders
Graduated cylinders are designed to deliver a volume of liquid. While
graduated cylinders also come in a variety of sizes, you will use primarily
the 10 mL and 50 mL sizes in lab. The precision of the volume
measurement is estimated to one tenth of the smallest division shown on
the cylinder. This will give you an additional estimated decimal place
(example 45.3 mL, never 45 mL). Most graduated cylinders in the lab
can be read to the tenths place ( + 0.1 mL)
Graduate cylinders are considerably smaller in width than in length (see
Figure 3) so the meniscus can be more easily seen and the spacing between
marks on the scale is larger.
1-2 cm in width, 15-30
cm in length
Graduated Cylinder
Figure 4
Pipets
Pipets (Figure 5) come in many sizes and many shapes, but if they are used for precise and accurate
measurement, they are made out of glass. Plastic pipets are disposable and are used only when exact
volumes of liquids are not required. Drawing in liquids to a pipet is the same, regardless of the type of
pipet. The pipet is designed to deliver a volume of liquid by gravity. Liquid is sucked into the pipet
using a pipet bulb or a pipet pump. The pipet is immersed in the liquid. The bulb is squeezed to expel
air, then gently, but firmly placed over the top of the pipet (the pipet is never inserted into the bulb). By
slowly and carefully releasing the pressure on the bulb, liquid will be sucked into the pipet. (Practice
this with water!) The appropriate amount of liquid is obtained and a finger (thumb or fore-finger) is
placed over the top of the pipet to hold the liquid at a certain volume, then the liquid is dispensed by
gravity into the appropriate receptacle.
For volumetric pipets: Volumetric pipets are designed to measure one volume and one volume only,
and are referred to as ‘to delivery (TD)’. All of the liquid in the pipet is allowed to drain by itself. There
should be a small amount of liquid left in the tip of the pipet – this last bit of liquid is never “blown” out
of the pipet by the bulb. The pipets are all calibrated to leave a small amount of liquid behind.
For graduated pipets: Graduated pipets are designed to measure a variety of volumes; they measure
volumes by difference. This means that they should never be allowed to drain completely; rather they
must be started and stopped and the interval determines the amount dispensed. Typically, the liquid
with withdrawn to the ‘zero’ mark, a finger is placed on the top of the pipet to hold the liquid and then
the liquid is slowly dispensed (by releasing pressure with the finger) until the desired volume is released.
The flow is then stopped again with a finger, and the exact volume dispensed is read off of the pipet.
Based on the markings, all pipets must be read to the hundredths place.
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Figure 5: Volumetric and graduated glass pipets
Discussion of Mass Measurements
In the chemistry lab, mass measurements are made using digital balances. Like glassware, these
balances can measure to different precision. Many digital balances can also measure mass in different
units (grams, ounces, etc…). In CHEM& 151 and 152, you will be using ‘top-loading’ balances, which
have a precision to the hundredths place. Be sure that the balance is set to the correct units before
measuring a sample. This precision is indicated by stating the balance is ‘good to ±0.01g’, meaning
that the balance gives measurements to two decimal places.
If smaller quantities or more precision is necessary for an experiment, an analytical balance may be
employed. This is also a digital pan balance, like a top-loader. However, it also has side and top shields
to isolate the pan from air currents that can affect the measured mass. An analytical balance measures
masses to ±0.1 mg, or four decimal places.
DENSITY DETERMINATION
Density is defined as mass per unit volume. There are a variety of different methods to determine the
density of a substance; the method chosen depends largely on the phase of substance and the shape of
the substance. In this experiment you will investigate two methods: direct measurement and volume
displacement.
Measuring the Density of a Liquid
Measuring the density of most liquid samples is fairly straightforward. A measured volume of the liquid
is weighed directly on a balance. The density is then directly calculated from the mass and volume
measurements.
Measuring the Density of a Solid
It is possible to determine the density of any solid regardless of shape by measuring its volume by water
displacement. The mass of the object is measured with a balance. The object is then submerged in a
precisely-measured volume of water. As the water level rises, the difference in volume represents the
volume of the solid object. The density is then calculated from the mass and this displaced volume.
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EXPERIMENTAL PROCEDURE
This experiment consists of three activities:
Activity 1: Equipment Scavenger Hunt
Activity 2: Determination of the Density of a Solid
Activity 3: Investing the Precision of Volumetric Measuring Devices
ACTIVITY 1: FINDING EQUIPMENT IN THE LAB: A SCAVENGER HUNT
This activity must be completed in the lab to obtain a final stamp.
While we encourage everyone to have a lab partner, this section will serve you better if it is completed
independently. If you do have a partner, make sure that each person looks around the lab and
participates in finding the items in the “scavenger hunt”. You need to find the items listed below (they
will be located in a variety of places around the lab!), and place them on the bench-top in front of
you in the order listed. After you have found all of the items, ask the lab instructor to check off on
your items. Once the instructor has given his/her approval, you should return the items back to the
appropriate place in the lab (note: some of the items you will need for the next section, so read that and
keep those items at your bench-top. Those can be returned after you finish the rest of the experiment!)
 By instructor
_______
_______
_______
_______
_______
_______
_______
_______
_______
_______
_______
_______
Item
Utility clamp
Buret Clamp
Buret
100-mL beaker
125-mL Erlenmeyer flask
Thermometer
Tongs
Ring stand
Test tube
Spatula/scoopula
10-mL Graduated pipet
25-mL Graduated cylinder
Instructor Initials:
__________________
You will need the 100-mL beaker and 25-mL graduated cylinder for the experiment, so you do not need
to put them away at this time.
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ACTIVITY 2: DETERMINATION OF THE DENSITY OF AN UNKNOWN SOLID OBJECT
PROCEDURE:
Do all your mass measurements on only one electronic balance – use the same balance for all mass
measurements.
1. Obtain one solid sample from the reagent bench and record the object identification in the data
table provided.
2. Using an electronic ‘top-loader’ balance, determine the mass of the solid, and record your mass
in the data table to the appropriate precision.
3. Obtain a 50-mL graduated cylinder. Add roughly 30 mL of water to your 50 mL graduated
cylinder; carefully record this volume to the proper precision.
4. Carefully add the solid object to the graduated cylinder. This should be done by tilting the
graduated cylinder at an angle and allowing the solid to slide down into the cylinder. If you
simply drop the object into the graduated cylinder you will either break the graduated cylinder or
splash water out, which makes your volume measurement inaccurate. The measured volume of
water should completely cover the immersed object and the graduated cylinder must be large
enough to produce measurable results.
Data Table 1: Density of a Solid Object
This table must be completed in lab to earn a final stamp.
Object Identification Letter
Mass of Solid Object
Volume of Object + Water
Initial Volume of Water
Volume of Solid Object
Calculated Density of the
Object (show calculation below)
Density Calculations
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Questions: (may be completed at home or in lab)
1. Consider the density of your object. Compare your calculated value to the values in the table you
completed in the prelab.
a. Based on this comparison, what is the chemical identity of your object?
b. Calculate the percent difference between your experimentally-determined density and the
accepted value. Show your calculation below. Pay careful attention to significant figures!
ACTIVITY 3: Investigating the Precision in Volumetric Glassware
Review the glassware section of this laboratory handout to remind yourself about significant
figures and measuring using these pieces of glassware.
Obtain the following pieces of glassware and equipment:
• Four beakers that are 100-mL or 150-mL in size; they do not all need to be the same size, but at
least one should have markings.
o Label these beakers 1, 2, 3 and 4 with tape.
• One 25-mL graduated cylinder.
• One 25-mL graduated pipet. (These will be on the reagent bench).
• A larger beaker (400 or 600 mL) with approximately 300 mL of distilled water.
PROCEDURE
Beaker Measurements
1. Take one beaker with markings; make sure it is clean and dry. Weigh this beaker empty, and
record its mass in the data table below.
2. Add approximately 20 mL of water to this beaker, and record the exact volume of water under
‘Trial A’ to the appropriate precision. (Example
3. Weigh the beaker with water and record the mass. Use this mass and the mass of the empty
beaker to determine the mass of water in the beaker.
4. Add an additional 20 mL of water (for a total of 40 mL) to the beaker, recording the volume and
mass in the table below under ‘Trial B’
5. Repeat the process two additional times, so that you record the volumes and masses of 60 mL
and 80 mL of water (Trials C and D).
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Data Table 2: Density of Water Measured by a Beaker. (must be completed in lab)
Mass of
Empty
Beaker
Trial
Volume of H2O
Mass of Beaker + H2O
Mass of H2O
A
B
C
D
Before going any further – obtain instructor initials _______________________________
Graduated Cylinder Measurements
1. Use all four beakers for this portion of the experiment; make sure they are all clean and dry, and
that they are clearly labeled 1 – 4.
2. Weigh each beaker on a balance, record the mass of the beakers in the table below.
3. Using a 25-mL graduated cylinder, measure 10 mL of water, and record the exact volume of
water in the data table below. Transfer the water to the beaker labeled 1, then, weigh the beaker
with water and record the mass. Use this mass and the mass of the empty beaker to determine
the mass of water in the beaker.
4. Repeat the process three additional times (trials 2, 3 and 4), so that you record the volumes and
masses of 15 mL, 20 mL and 25 mL of water.
Data Table 3: Density of Water Measured by Graduated Cylinder. (must be completed in lab)
Trial
Volume of H2O
Mass of Beaker +
H2O
Mass of Empty
Beaker
Mass of H2O
1
2
3
4
Before going any further – obtain instructor initials _______________________________
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Pipet Measurements
1. Use all four beakers for this portion of the experiment; make sure they are all clean and dry, and
that they are clearly labeled 1 – 4. Weigh each beaker on a balance, record the mass of the
beakers in the table below. (Be sure to weigh each beaker again, do not use the masses from the
previous measurements)
2. Practice drawing water into the 25-mL graduated pipet and controlling the flow of water from
the pipet.
3. Using a 25-mL graduated pipet, draw in enough water to fill the pipet to the zero mark. Into the
beaker labeled ‘1’, dispense 10 mL of water, and record the precise volume of water in the data
table below. Weigh the beaker with water and record the mass. Use this mass and the mass of
the empty beaker to determine the mass of water in the beaker.
4. Repeat the process three additional times (trials 2, 3 and 4), so that you dispense and record the
volumes and record the masses of 15 mL, 20 mL and 25 mL of water.
Data Table 4: Density of Water Measured by Graduated Pipet. (must be completed in lab)
Trial
Volume of H2O
Mass of Beaker +
H2O
Mass of Empty
Beaker
Mass of H2O
1
2
3
4
Before going any further – obtain instructor initials _______________________________
Processing the Data
(This section may be completed in the lab or at home. It is not required for the final stamp.)
For each measuring device (the beaker, graduated cylinder and graduated pipet), you will need to
determine the average density of water; this will be done graphically. Refer to the Graphing handout in
your lab packet and the “Representing Data and Results Using Graphs” in your Laboratory Handbook.
Graph the mass of water versus the volume for each device, add a linear trendline to the graph and
display the equation of the line along with the R2 value (the correlation factor) on the graph. Print out
each graph separately. Use your graphs to complete Data Table 5 and answer the following questions.
When reporting your densities, think about how many significant figures you should report for your
values!
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Data Table 5: Average Densities of Water Based on Measuring Device.
Measuring Device
Density of Water
Correlation Factor (R2)
Beaker
Graduated Cylinder
Graduated Pipet
Questions:
(This section may be completed in the lab or at home. It is not required for the final stamp.)
1. A reasonable assumption given the water’s temperature is that the density of water is 0.998 g/mL.
For each of your devices, compute the percent difference between your average density value for
each piece of glassware and the true density. Show your work and pay careful attention to the
significant figures when you perform the subtraction!
Percent Difference =
accepted value - experimental value
x 100%
accepted value
Beaker:
Graduated cylinder:
Pipet:
2. Which volume-measuring device was easiest to use, and why?
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3. Which device was the most accurate? Which was the most precise? Explain how you arrived at
these conclusions.
4. Based on the prelab, the pipet was the measuring device that can be read with the greatest precision
(the greatest number of decimal places). Was the pipet the most precise tool based on your data?
What other factors might affect the precision of a measuring device?
5. Why do you use the slope of the line instead of the individual data points to determine the density?
Explain your answer by choosing one of your data points, calculating the density and making a
comparison.
6. A student in another chemistry class needs an accurate way of determining the density of a liquid,
and comes to you seeking guidance. Which technique would you recommend to the student to use?
What advice would you give to the student to help them with this technique?
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Stamp:
Basic Equipment PRELAB
Name __________________
Prelab Questions: These questions are to be answered before you come to the laboratory. Always
read the experiment before starting the prelab. Your textbook, Lab handout (this packet) and
the Laboratory Handbook are excellent resources. It is your responsibility to get this stamped by
the Lab Instructor before you begin working on the experiment!
1. How many digits after the decimal place can you read a (circle one):
a) beaker
0
1
2
b) graduated cylinder
0
1
2
c) pipet
0
1
2
3
3
3
2. You have a sample of titanium metal. You mass it on an electronic balance and it has a mass of 7.52
grams. You decide to measure the volume of the sample by volume displacement. You fill a
graduated cylinder with 35.2 mL of water. After dropping the titanium into the graduated cylinder,
the volume now reads 36.9 mL. Determine the density of titanium:
Density =
If the density of titanium is 4.51 g/mL, what is the percent difference between your value for density
calculated above and the accepted value for the density? Show your calculations. (see formula on pg 3)
Percent difference =
3. Look up the densities for the following pure elements. You can look them up in your text, in a
handbook (available in the library), or online, but you must cite your source. A space is provided
for the citation below the table.
Element
Density (g/mL or g/cm3)
Element
Iron (Fe)
Zinc (Zn)
Nickel (Ni)
Copper (Cu)
Aluminum (Al)
Titanium (Ti)
Density (g/mL or g/cm3)
Citation:
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4. The following data was collected by using two different types of glassware to measure the volume.
The data was graphed with the masses on y-axis and volumes on the x-axis.
Glassware A
Glassware B
Mass of Water (g)
Volume of Water (mL)
Mass of Water (g)
Volume of Water (mL)
10.01
14.92
19.89
24.89
10.00
15.00
20.00
25.00
10.50
14.86
20.23
25.12
10.0
15.0
20.5
25.0
Use the graphs below to answer the following questions.
Glassware A:
30.00 25.00 20.00 15.00 10.00 y = 0.9922x + 0.0640 R² = 0.99998 5.00 0.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 Glassware B:
30.0 25.0 20.0 15.0 10.0 y = 1.0246x ‐ 0.4881 R² = 0.99787 5.0 0.0 0.00 #2 Basic Equipment
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a. Label the axes and put appropriate titles on the graphs. See the ‘Graphing Techniques’
document in your lab packet for assistance.
b. What are the units on the slope for the best-fit lines?
c. What physical property is represented by the best-fit lines?
d. Report the slopes of the best-fit lines, with correct significant figures and units.
e. Which piece of glassware is the most precise? How did you make this decision?
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