CHEM 1P91 Experiment 1 Density Determination
Summary
TLDRIn this pre-lab talk for M1 p91 labs, the instructor introduces a video-based approach to pre-lab preparation, emphasizing its utility for both lab preparation and reference. The focus is on determining the density of aqueous ethanol solutions, with students tasked to ascertain the ethanol percentage in their assigned unknown sample. Key points include the importance of the lab manual, the use of calibration data for plotting a quadratic line of best fit, and the experimental procedure involving measuring volume and mass to calculate density. The talk also covers the handling of equipment, the significance of accuracy and precision, and the differentiation between random and systematic errors. Lastly, the instructor reminds students of the need for TA verification on data sheets and the importance of uploading signed documents.
Takeaways
- 📚 The pre-lab talk is recorded as a video and posted on Brightspace to supplement the lab manual, not replace it.
- 🔍 Students are responsible for determining the percentage of ethanol in their assigned unknown sample, labeled A to E.
- 📊 A calibration graph using the provided data table is required, with a quadratic line of best fit rather than a linear one.
- 💧 The density of the unknown sample is measured using a burette for volume and an analytical balance for mass.
- 🔑 The key to finding the percentage of ethanol is using the average density in conjunction with the equation from the quadratic fit.
- 📈 Desmos is recommended as a free plotting tool for creating the calibration graph, with YouTube tutorials available for guidance.
- 🏷️ Graphs must be clearly labeled with axes, a title, and the unknown sample labeled after plotting.
- ⚖️ The experiment involves measuring mass and volume to calculate density, emphasizing the importance of accuracy and precision.
- 🌡️ Caution is advised when handling the flask to avoid heating it, which can cause evaporation and affect the mass reading on the balance.
- 📋 TA's signature and date are required on data sheets, and original signed sheets must be uploaded by the student.
Q & A
What is the main purpose of the pre-lab talk video?
-The main purpose of the pre-lab talk video is to supplement the lab manual by highlighting key points and providing guidance for the lab, which helps students prepare and also serves as a reference for completing the lab report.
What is the first experiment mentioned in the script?
-The first experiment mentioned is the determination of density for aqueous ethanol solutions.
How many unknown samples are there in the first experiment?
-There are five unknown samples labeled A to E in the first experiment.
What is the range of ethanol percentage in the unknown samples?
-The unknown samples could have anywhere from 0 to 90% ethanol.
What method is used to determine the percentage of ethanol in the unknown samples?
-The density method is used to determine the percentage of ethanol in the unknown samples.
What type of graph should be used to plot the calibration data for density versus ethanol percentage?
-A quadratic line of best fit should be used to plot the calibration data since the relationship is not linear.
What tool is recommended for plotting the calibration data?
-Desmos is recommended as a free online tool for plotting the calibration data and obtaining a quadratic line of best fit.
What equipment is used to measure volume and mass in this experiment?
-A burette is used to measure volume, and an analytical balance is used to measure mass.
How should the flask be handled to minimize errors?
-The flask should be handled by holding it with a few fingers near the top to avoid heating it up, which can cause evaporation and affect the mass measurement.
What is the difference between accuracy and precision as discussed in the script?
-Accuracy refers to how close the experimental results are to the actual value, while precision refers to the reproducibility and consistency of the results.
Why is it important to perform multiple trials in this experiment?
-Multiple trials are important to reduce random errors and increase precision by averaging the results.
What are the two types of errors that are not considered experimental errors in the lab report?
-Personal errors, such as spilling a sample or making a procedural mistake, are not considered experimental errors.
Outlines
📚 Introduction to M1 p91 Labs and Pre-lab Talk
The instructor welcomes students to the M1 p91 labs and explains that due to the current circumstances, pre-lab talks will be delivered as recorded videos on Bright Space instead of in-person. These videos will supplement the lab manual and provide key points and comments to help students prepare for the lab and complete their lab report sheets. The first experiment involves determining the density of aqueous ethanol solutions. Students will be given an unknown sample labeled with a letter and must determine the percentage of ethanol in it. The lab manual will be essential for understanding the procedure, as the video will not cover every detail. The instructor emphasizes that the unknown sample's ethanol percentage could range from 0 to 90%, and it's not necessarily listed in the table provided in the manual. This table will be used for calibration purposes.
🔍 Experiment Details and Equipment Usage
The instructor outlines the procedure for the density determination experiment, which involves measuring the volume and mass of the unknown sample to calculate its density. Students will use a burette for volume measurements, an analytical balance for mass, and a 25 mL flask for sample handling. The process of weighing by difference is explained, where the initial and final masses of the flask are measured to determine the mass of the sample. The instructor advises on how to save time by using the final mass of one trial as the initial mass for the next. The importance of handling the flask correctly to avoid errors due to heat is emphasized. The concept of experimental errors is introduced, distinguishing between personal errors, random errors, and systematic errors. The instructor stresses the importance of accuracy and precision in the lab work, explaining the difference between the two and how they relate to the quality of the experimental results.
🎯 Accuracy, Precision, and Error Analysis
The instructor discusses the concepts of accuracy and precision, using a target analogy to illustrate the difference. Accuracy refers to how close the experimental results are to the actual value, while precision is about the reproducibility of the results. The instructor explains that multiple trials help reduce random error and increase precision. Systematic errors, which are consistent and affect accuracy, can be due to poor technique or instrument issues. The importance of proper lab technique to minimize errors is emphasized. The instructor also reminds students of the requirements for lab reports, including getting their TA to sign and date data sheets and uploading the signed observation sheets.
Mindmap
Keywords
💡Density
💡Aqueous Ethanol Solutions
💡Pre-lab Talk
💡Lab Manual
💡Unknown Samples
💡Calibration Data
💡Quadratic Line of Best Fit
💡Desmos
💡Weighing by Difference
💡Analytical Balance
💡Experimental Errors
Highlights
Introduction to M1 p91 labs and the shift to pre-recorded pre-lab talks posted on Bright Space.
Pre-lab talks are supplementary to the lab manual and do not replace the detailed procedure descriptions.
The first experiment involves determining the density of aqueous ethanol solutions.
Students will be assigned an unknown sample with a varying percentage of ethanol to identify.
A calibration table from the lab manual is used to plot density against ethanol percentage for system calibration.
Students must use a quadratic line of best fit, not a linear one, for plotting due to the nature of the data.
Desmos is recommended as a free online graphing tool for plotting and finding the quadratic line of best fit.
The density method is used to determine the percentage of ethanol in the unknown sample.
Instructions on how to handle the 25 mL volumetric flask to avoid errors due to heat and evaporation.
The importance of recording the initial mass of the flask for accurate mass measurement by difference.
Procedure for conducting the experiment, including measuring volume and mass to calculate density.
Advice on how to save time by using the final mass of one trial as the initial mass for the next.
The calculation of density by dividing mass by volume and the necessity of performing the trial multiple times for better averages.
Instructions on how to plot the graph with proper labeling, title, and visibility for uploading with the lab report.
Differentiation between accuracy and precision in experimental results and their significance.
Explanation of random error affecting precision and systematic error affecting accuracy.
Advice on proper lab technique to minimize errors and improve the quality of experimental results.
Requirement for students to get their TA to sign and date data and observation sheets and to upload them.
Final encouragement and reminder to follow the lab procedures carefully for successful outcomes.
Transcripts
welcome to the M1 p91 labs for this term
as I mentioned in lecture I'm posted on
bright space instead of doing an
in-person prelab talk this year what
I'll be doing instead is recording my
preab talk and posting it as a
video um so this will be helpful not
only because it helps you to prepare for
the lab but you can access it afterwards
as well uh there will be a lot of
information in these videos
that you can use to answer some of the
questions on the U lab report sheets
that you have to hand in so just a word
of caution before I start these pre-lab
talks are meant to supplement the
description in your lab manual they're
not a replacement for that I will not be
going through every detail of the
procedure I'm going to highlight some of
the key points make some comments uh but
you will still need to read the
procedure in the manual uh since most of
the details for the actual experimental
steps uh you'll find
there so the first experiment is the
determination of density specifically
for aquous ethanol
Solutions if you took chem 1p you may
recall that our first experiment was a
density experiment this Builds on that
so for this experiment there are five
unknown samples labeled a to e and
before you start the lab I will uh give
you you an unknown letter that will be
your unknown uh and uh the purpose is
for you to determine what the percent
ethanol is in your unknown uh make sure
you record the letter I give you so that
we know which one you have and the
unknown could have anywhere from 0 to
90%
ethanol um you don't know what it is so
you have to find out and we're going to
use the density method to do so um now
the just the word uh caution uh in the
past some students thought that their
unknown had to be one of these that you
see in the table uh on the slide table
one it doesn't necessarily have to be
that it could be anywhere between 0 and
90% not necessarily one of the ones you
see on this
table so um this table I took directly
out of your manual it's on the first
page of the experimental description and
it is your calibration data um you'll
use this in order to calibrate uh the
behavior of uh density versus uh ethanol
percentage for the system and find out
what the percent ethanol is for your
system uh so what you you'll do is uh
make a plot of this table make a graph
of it with uh the density on the x- axis
and percent ethanol on the y- AIS and um
I'll let you know ahead of time that it
is not a line that is linear so don't
try to force a yal MX plus b fit on it
you'll have to use a quadratic line of
best fit now that sounds difficult but
any plotting routine that you have will
do that for you uh if you do not have a
uh plotting uh app that you paid for you
can find the free one online called
Desmos um many high schools use that
it's free uh you can find it online d m
oos and uh there's several YouTube
videos very short and good YouTube
videos to show how to use Desmos
including how to get a quadratic line of
best fit so you'll end up with this
equation which has two variables the
percent uh ethanol which will be the Y
variable and the density which will be
the X variable so what you do is
determine the density for your unknown
and then use the equation to get the
percent ethanol for your unknown
um and don't forget to label your
unknown on the graph once you've plotted
it make sure the graph is uh you know
large and visible uh you're going to
have to upload it don't make a tiny
little graph that the TA can barely see
use a whole page and you'll be uploading
it along with your lab report um and uh
don't forget standard things for making
graphs such as labeling the axes make
sure there's a good title and so on
um okay so there isn't a lot of
equipment to use for this experiment
you'll be using a buet to measure volume
the analytical balance to measure mass
and your sample uh is going to be
handled using this small uh 25 mil llin
Meer flask uh you weigh by difference uh
meaning that you will weigh the flask
initially then you go and put in about
uh 10 to 12 milliliters of uh of your
sample you will of course have to
measure exactly how much but it doesn't
matter uh how much it is between 10 to
12 as long as you know to two decimal
places what volume you delivered so you
measure the initial volume on the buet
the final volume the difference is the
volume that you dispensed into the flask
then you go and weigh the flask a second
time and the difference is the mass
you'll weigh uh the Vol the sorry you'll
weigh the flask to four decimal places
on the analytical balance
now after you've done the first trial
you don't have to empty the flask and uh
you know dry it and uh do it again by
weighing at the beginning um you need to
know what the initial mass of the flask
is so it doesn't have to be empty and
dry as long as you know what the flask
weighs to begin with that's all that
matters so what you can do to save time
is after you've done your first trial
you can use the final mass of trial one
and write it down immediately as being
the initial mass for trial two that's
why your flask weighs now with some uh
some liquid in it and then go and
dispense more and weigh the final volume
Etc okay um from the mass and the volume
mass divided by volume is the density
you do that three times to get a better
average and from the average density you
use your equation of best fit in order
to find the percent Ethan
um when you handle the flask try not to
hold it uh and you know too much with
your hand uh I would use uh just a few
fingers near the top don't put the whole
palm of your hand around the flask the
reason is because you'll heat up the
flask especially if you're waiting in
line uh to use the analytical balance
you'll heat up the flask by holding it
with your hand all around it and that
creates two errors one error is that uh
if the flask warms up there's
evaporation happening from the liquid
anyway which is an error because if some
of the liquid evaporates you'll lose
some of the mass but if you warm it up
that increases the amount of evaporation
so that's one thing the other thing is
that a warm or hot object when you weigh
it on analytical balance weighs slightly
less and the reason is because it warms
up the air around it and there's less
air pressing down on the balance so um
make sure you use that uh flask in the
way I've mentioned just hold it with a
couple of fingers near the
top okay um I mentioned error and so
you'll always be asked for experimental
errors in an experimental writeup uh
don't put down personal errors you know
things like uh I spilled I I might have
spilled some sample or I made a mistake
in doing this step personal error are
not to be recorded or uh stated in the U
lab report as experimental errors what
we're looking for are two other types of
Errors namely uh random error and
systematic error and a little bit more
about that on the next
slide okay so the experiment is pretty
straightforward you know you just weigh
the mass of a sample mass divided by
volume is density so it's not
theoretically very difficult however you
are being tested on accuracy and
precision and this is one of the main
reasons sometimes people get low marks
on the lab report so accuracy and
precision are not the same thing
sometimes we use the words
interchangeably in everyday language
they are very different uh this picture
on the left shows a Target and you're
trying to hit the target and uh that
represents you know each point each red
Point represents a experimental trial
but you can apply this to anything
including uh trying to shoot at a Target
and so on so accuracy is how close you
are to the bullseye the center of that
Target and in the experiment that would
be how close you are to the actual value
so um I know what the percent alcohol is
for your unknown sample I made the
solutions how close you report the
accuracy how close it is to the actual
value is
accuracy so um how close your value is
as to the actual value and one trial can
determine that if you only were limited
to doing one trial you can get a number
and see how close you are to the real
value Precision has to do with
reproducibility you do the experiment
more than once how close are those
values to each other since you're
repeating the same experiment you should
get in theory identical values but
because of error there's
fluctuations and precision is a measure
of those fluctuations the closer the
numbers are to each other the more
precise you are so if you look at the
picture on the left on the top left
corner you'll see an example of what is
not accurate you're offt Target but
you're precise because the numbers are
all very close to each other on the top
right you have both accuracy and
precision because not only are you on
target but you're on target pretty much
every time with only a little
fluctuation bottom left very inaccurate
very imprecise the numbers are all over
the place
and uh you're off Target on the right
you're still off Target but you're a
little closer so you might call that
accurate especially if you average those
and the average you'll see uh will get a
little closer uh to the Target in fact
that example um shows how it shows why
we do multiple trials uh multiple trials
allow us to average and reduce the error
in Precision because if you take those
four values on the bottom right and take
the average since some are higher some
are lower some are to the left some are
to the right some of the error cancels
and so the average will actually be
closer to the
Target on the right you see the
connection between accuracy and
precision and between uh systematic and
random error so if you look at the top
Target that's a representation of very
little or no error there's always a
little bit of error but there's very
little error there you're on Target and
with very little flu situation You're On
Target every time um so that's both
accurate and precise in the middle
Target uh you're close to the bullseye
which means you're being accurate
especially if you average those numbers
but the individual numbers fluctuate by
quite a bit and so you're not being as
as
precise uh on the bottom you have an
example of uh not being accurate of
Target but you're being precise uh
because you're reproducing it every time
how this relates to error is as follows
random error as the name implies
sometimes gives you a result higher or
lower when you repeat the experiment
sometimes you get a number that's higher
sometimes you get a number that's lower
that's why we do multiple trials and
average so the average eliminates some
of the random error so random error
relates to Precision systematic error
relates to accuracy uh systematic error
is always off by the same amount the
same direction like a clock that's 10
minutes fast for example and systematic
error can point to both a bad technique
on your part or it often uh um is a
reflection of a broken instrument if the
analytical balance is broken or not
calibrated properly it gives you a
number that's off in the same direction
uh by the same amount like a clock
that's too fast or too
slow whereas random error reflects uh
Precision uh you can see why by looking
at the middle uh the more random the
numbers are the more imprecise are the
results and that usually is a reflection
of lab technique so this experiment uh
by by giving you a mark for accuracy and
precision we're essentially testing you
on your accuracy and precision so
although the experiment is theoretically
simple be careful with your Technique
because it will affect your accuracy and
and your precision
uh okay so that's it um just a couple of
last uh things don't forget that you
have to get your ta to sign and date to
your data and observation sheets it's
your responsibility to make sure they do
that and you leave them with a hard copy
and don't forget that you must upload
the original uh signed data and
observation sheets you have to upload
that your ta will not do it for you even
though they have a copy they mark them
online and they're not going to take
pictures of 80 different students that
they have and upload those for them and
I won't be doing it either if you email
it to me you have to remember to upload
your data and observation sheets that
are signed by your ta okay uh all right
good luck
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