CHEM 1P91 Experiment 1 Density Determination

Summary

In 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

00:00

📚 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.

05:01

🔍 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.

10:04

🎯 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.

Keywords

💡Density

Density is defined as the mass per unit volume of a substance. In the context of the video, density is a key physical property utilized to determine the percentage of ethanol in aqueous solutions. The video emphasizes the importance of accurately measuring both the mass and volume of the samples to calculate density, which is then used to infer the ethanol concentration.

💡Aqueous Ethanol Solutions

Aqueous ethanol solutions are mixtures of ethanol and water. The video's main experiment revolves around determining the ethanol concentration in these solutions. Understanding the properties of these solutions, such as how their density changes with varying ethanol percentages, is crucial for the lab work described.

💡Pre-lab Talk

A pre-lab talk is a preparatory discussion or lecture given before conducting a laboratory experiment. In the video, the instructor mentions replacing in-person pre-lab talks with recorded videos, which serve to supplement the lab manual and provide key insights for students to prepare for the lab session effectively.

💡Lab Manual

The lab manual is a document that provides detailed instructions and procedures for conducting experiments. The video script cautions that the pre-lab talk is not a replacement for the lab manual; students are expected to read the manual to understand the full procedure, as the talk only highlights key points.

💡Unknown Samples

In the context of the video, unknown samples refer to the five aqueous ethanol solutions labeled A to E, whose ethanol concentrations students must determine. The term 'unknown' signifies that the exact percentage of ethanol in these samples is not disclosed to the students initially and is part of the experimental challenge.

💡Calibration Data

Calibration data are used to standardize the relationship between two variables, in this case, density and ethanol percentage. The video instructs students to use a table from the lab manual as calibration data to plot a graph and find a quadratic line of best fit, which will be used to determine the ethanol percentage from the density of their unknown samples.

💡Quadratic Line of Best Fit

A quadratic line of best fit is a type of regression analysis used when the relationship between variables is not linear. The video emphasizes that students should not force a linear fit on the data but instead use a quadratic fit, which is more appropriate for the non-linear relationship between density and ethanol percentage.

💡Desmos

Desmos is a free online graphing calculator mentioned in the video as a tool for plotting graphs and finding a quadratic line of best fit. The video suggests using Desmos for students who do not have access to a paid plotting application, highlighting its utility in the lab experiment for data analysis.

💡Weighing by Difference

Weighing by difference is a method used to determine the mass of a substance by weighing the container before and after adding the substance. The video describes this technique as part of the procedure for measuring the mass of the ethanol solution, emphasizing the importance of knowing the initial mass of the flask for accurate calculations.

💡Analytical Balance

An analytical balance is a precise instrument used for measuring the mass of substances to a high degree of accuracy. In the video, the analytical balance is mentioned as a critical piece of equipment for accurately weighing the flasks to determine the mass of the ethanol solutions.

💡Experimental Errors

Experimental errors refer to the discrepancies between the measured and true values in an experiment. The video distinguishes between random and systematic errors, explaining that random errors affect precision (reproducibility of results) while systematic errors affect accuracy (closeness to the true value). Understanding and minimizing these errors is a key aspect of the lab work.

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