Introduction
Purpose
Labworks II
Data Sheet
Procedure
Conclusion
Volumetric Glassware
Pre-Laboratory
Assignment
As a youth raised in the rural Midwest, one of the pastimes in late summer
was helping (hindering) my mother and grandmother can vegetables
harvested from the garden. My favorite was sweet chunk pickles. As an adult,
when we had our own family garden, my first canning effort was those
favorite sweet chunks! My mother passed on the recipe that had been in our
family for generations. Here is that fabulous recipe that won blue ribbons at
our local county fair!
•
2 gallons of fresh cukes
•
Salt
•
Alum, size of a walnut
•
6 pints of sugar
•
1, 5-cent pkg. of cinnamon sticks, broken up
•
2, 3-cent pkgs. of celery seeds
•
½ gallon- 100% apple vinegar
Wash cukes; place in stone jar. Cover with salt water, using enough salt to each gallon of
water to float an egg. Cover with fresh grape leaves, let stand 1 week. Remove the cukes from
the brine; cover with boiling water. Let stand 24 hours. Remove pickles; cut into chunks.
Cover again with boiling water and add alum. Let stand 24 hours. Remove chunks. Combine
sugar, cinnamon sticks, celery seeds, and vinegar and bring to a rolling boil. Pour the boiling
hot syrup over the chunks; for the next four days reheat the syrup and pour over chunks.
The fifth day, can using quart mason jars that have been well washed.
Part of the secret to this recipe other than trying to figure out, is that an English or a black
walnut- how many cinnamon sticks were in a 5-cent package-how many ounces of celery seed
to use- do I use free-flowing iodized or noniodized salt, was to have the correct brine solution.
The traditional test was to try to just float a raw egg in the brine solution. If the egg sank, the
concentration of the salt in the solution was too low. If the egg floated too high, the
concentration was too great, and water was added before adding the cukes. (What difference
does it make anyway? Will the pickles really be mushy or crisp?)
Science to the rescue (Use of Volumetric Glassware)
Chemists use special volumetric glassware in the chemistry laboratory
(rather than the ordinary beakers and flasks) when they need highprecision measurements of the volumes of liquids. An understanding of the
proper use of and the limitations of such glassware is important in
achieving reliable results. One of your personal goals for this course is to
develop sound experimental techniques.
In this experiment you will practice using various volumetric glassware to
improve your technique and to solve an important historical problem in
practical chemistry.
A. Volumetric Glassware
Glassware such as Erlenmeyer flasks, beakers, and test tubes are NOT
calibrated for use as accurate measuring devices for liquids. Graduated
cylinders are somewhat more accurate, burettes if used properly are quite
accurate. We recommend the use of volumetric flasks or of the calibrated
pipets. Volumetric glassware is designed for two quite different purposes.
One is used to prepare specific volumes of solutions; the most common is
the volumetric flask.
graduated cylinder
volumetric flask
Volumetric flasks should be used as the container when the concentration of the solution
being prepared must be very accurate (3-4 sig. figs). Volumetric flasks are available in a
variety of sizes; 5,10, 25, 50, 100, 250, 500, 1000 mLs.
The second type of volumetric glassware is used to transfer precise
volumes of liquid; pipets are the standard glassware used by the chemist.
The standard calibrated glass pipets come in basically two styles. The first
style of pipets are constructed to deliver repeatedly a precise specific
volume of liquid. These pipets discernible by the TD (to deliver) mark
etched in the glass. For TD glassware, the volume released is exactly the
volume marked on the pipet. The instructions for filling and emptying a
pipet are given at the end of the laboratory procedure. Some liquid will
remain in the pipet when delivery is complete. Do not remove the
remaining liquid in the tip of the pipet. The second style of pipets (Mohr) is
constructed with precise calibrations on the side of the glass. The
graduations on the pipet end before the tip therefore the volume of liquid
in the tip of the pipet is NOT known.
Pipet, Measuring (Mohr)
Pipet, Volumetric, Calibrated to Deliver (TD)
The discipline of chemistry is based on experimental observation. Many of these
observations require making repeated measurements. After making the measurement three
or four times, one normally compares the result of these measurements. This is done by
determining first the deviation of each measurement and then calculating the standard
deviation.
Mean,
,The arithmetic average of all measured values where
value and N equals the number of measurements.
ν1 the observed or measured
Deviation describes how much each measurement differs from the average of all
measurements. Deviation provides an indication of the quality of the measurements.
Deviation is calculated by:
|ν -
|
where is the mean is average value of the measurement and v is the
individual experimental value.
Standard Deviation, s, takes the deviation of all measurements into account
and is calculated by:
n = the number of values. The term n - 1 is referred to as the number of degrees of
freedom. The smaller the value of s, the more precise the data and the more confidence we
where
have in our work. How reliable is the analysis? To answer this question we compute the
probability that any given measurement will fall within certain limits around the mean. A
few probability values are given below and extensive tables are available in statistic books.
Thus if 1000 measurements are made, the probability is that 997 values lie within m 3s, 995
values lie within m 2s, and so on. The "95% confidence limits" are therefore the results that
range from -2s to +2s from the mean; only once in twenty times would a value lie outside
these limits.
Probability that a result will fall
within the limits, expressed as %
66.8
95.5
99.7
s or -s
+
1.0
2.0
3.0
The Floating Egg Experiment
A. Purpose:
Find the answers to the following questions:
1. What is the density (in grams per mL) of the "ideal" solution
that the canners were aiming to achieve?
2. In what way does the type of volumetric glassware used:
graduated cylinder, volumetric flask, Mohr pipet, volumetric
pipet, or buret effect the value and the precision of the calculated
density?
Make copies of the following :
Pre-Laboratory Assignment
Data Sheet
Procedure
Sodium chloride will be used in this lab. The solutions are still concentrated
enough to pose a hazard to the eyes. Goggles must be worn. Salt, eggs,
standard glassware, balances, and de-ionized water will be available. Other
items may be available upon request.
Clean and dry all glassware before use.
Preparation of the test solution.
You will prepare this solution in a team of four individuals.
Follow the following guidelines:
i Make at least 500-mL of the solution.
ii Use only one egg for testing.
iii When you have prepared the solution, each individual should record
the procedure in his (or her) notebook.
Collecting the experimental data.
The team should borrow a 50-mL buret; a 25-mL Mohr pipet; a 5, 10, 25, and
50-mL volumetric pipets; and a 25-mL graduated cylinder from the supply
area in the laboratory. (These must be cleaned and returned at the end of the
experiment.) Clean the borrowed equipment. Each person in the team will
work with one of the four types of equipment. Decide who will work with
which.
Note for future reference that when the net mass of something is determined
by subtracting the empty (tare) mass from the gross mass, the uncertainty
will be twice the sensitivity. This is because there is no guarantee that each of
the two mass readings will deviate from the true mass in the same direction
Now carry out four measurements where you use different volumes of
solution for each measurement. The volumes selected should range from 5
mLs to 25 mLs. Determine the weight of the flask + solution.
a. Buret
Make sure the stopcock on the 50 mL-buret is properly
assembled. Use a buret clamp and ring stand to mount the buret
vertically. Fill it to about the zero level with your solution. Drain
some out to insure there are no air gaps in the tip of the buret.
Record the level reading after it is stabilized. * The level should
be recorded to the nearest 0.01 mL. (Estimate the location of the
bottom of the meniscus between the calibration marks, which are
0.1 mL apart.) Deliver water from the buret into the clean, dry
Erlenmeyer flask. Deliver solution from the buret until you have
the desired volume. Record the volume to the nearest 0.01 mL.
Now weight the Erlenmeyer flask + solution and record the total
mass.
Repeat the entire procedure for a total of four trials. You want to
develop your technique so you can use a buret accurately and
reproducibly.
*
Taking a reading too quickly will not take into account the liquid that slowly
trails down the sides of the buret.
b. Mohr Pipet
Using the Mohr pipet, attempt to deliver exactly the desired
volume of solution to the dry Erlenmeyer flask. If you undershoot
or overshoot, record the actual volume you end up delivering.
Never apply suction by mouth to fill up this or any pipet. Do a
total of four trials. Attempt to perfect your technique.
c. Volumetric Pipet (5, 10, 15, and 25 mL)
Repeat the above weighing procedure using the four various sizes
of volumetric pipets to deliver desired volume of solutions into
the dry Erlenmeyer flask. The tips of the pipets are very fragile.
Be careful not to exert pressure on them. Do a total of four trials.
d. Graduated Cylinder
Fill your cylinder with the necessary volumes of solution and
record the "exact" volume. Mass the solution-filled graduated
cylinder and record the total mass.
Instructions for proper use of volumetric pipets and pipet bulbs.
C. Data Manipulation (to be completed during the laboratory period).
Using LabWorks II software program, which is introduced below, you will
enter your experimental data, then graphically determine the density of the
solution. Report your experimentally determined density and the y-intercept.
What is the signifigance of the value of the y-intercept. Attach the printout of
this information in your report. Be sure to record these values with the
correct number of significant figures.
1. Collect the density data from others in the class who used the
SAME type of glassware but different solutions. Determine the
mean and the standard deviation for this collection of data.
2. Collect the density data from members of your own team.
Determine the mean and the standard deviation for this
collection of data. All groups (same glassware and members of
your own team) must place on the blackboard their mean density
and standard deviation obtained with each type of glassware and
the freshness date of the egg used by the team.
Instructions for the use of LabWorks II
- Computer-based measurements (must be completed during the laboratory period).
i. Opening LabWorks II .
To open the LabWorks II program either,
Click on the Microsoft start button in the lower left corner of the
screen. A popup screen appears, click PROGRAMS, and then drag
the cursor over to LabWorks II 4. Another pop-up window will
appear; then click on LabWorks.exe. The LabWorks II program is
then activated.
or open LabWorks II 4 by clicking on the ICON found on the
Desktop Window.
A view of the LabWorks II the Main menu window should be on your screen.
ii. Spreadsheet Setup View.
The data you and your colleages have collected will be entered into a
spreadsheet. In this experiment, you will enter the data manually.
In the Main Menu screen on the Toolbar bar, open the Analyze window by clicking on the
button labeled Analyze.
View the Analyze Window
The Analyze window appears showing two of the possible five
views available. In this experiment, we will be using only the:
Spreadsheet view and
Graph view
We must accomplish three tasks:
Define Column A,
Define Column B,
Enter our Experimental Data
iii. Graph Setup View
In the Analyze window right click anywhere in the graph view area. A popup
window appears, select Properties.
The Graph Properties popup window appears. We will use this window to set
the properties for the graph.
We must accomplish six tasks:
Define the General appearence of the
Graph,
Define the appearence of the X-Axis,
Define the appearence of the Y1-Axis,
Define the appearence of the Y2-Axis,
Assign analysis of the Data,
Define the analysis of the the data, Curve
Fitting
iv. Statistics
You now will obtain both the mean and standard deviation for the
followingtwo groups
all students who used the same glassware
all sstudents using the same prepared solution as yours.
Collect the data for both groups. Make certain the Spredsheet and
Graph view is on the computer screen, in the Spreadsheet view on
column C.and enter the data into LabWorks II.
v. Print
Make certain the Spredsheet and Graph view is on the computer screen, to
print your graph, click on the File button found in the menu toolbar.
D. Conclusion (Individual)
In addition to any routinely required discussion assigned by your individual
instructor, discuss the following. In your conclusion, you should include the
numerical results. This should include an indication of the confidence of
your own experimental data. Also compare and contrast your data to those
obtained by the others in your team and by others in the room with the same
glassware. How did the amount of variation in your data compare to the
amount obtained by others with the same or with different glassware?
Finally, answer these questions in one or two sentences:
1. What was the density of the "ideal" solution"?
2. Talk with the members of your team and discuss each
member's measuring device. What were the
advantages/disadvantages of each measuring device? (Not just
the glassware you worked with but all of the glassware!).
Make copies of the following :
Pre-Laboratory Assignment
Data Sheet