Measurement and Density Lab

MEASUREMENT, DENSITY, AND GRAPHING
(adapted from ClarkCollege Chemistry course)
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. This first experiment will
introduce you to:
-Significant Figures
-Finding Equipment in the lab
-Proper methods for performing volume and mass measurements.
-Methods of measuring and calculating density of an unknown solid by volume displacement.
-Density and graphing
Significant figures in your measurements:
• Digital instruments (example balance): record every digit given. For example,
if the balance readout looks like this, record 1.70g, not 1.7g.
• Manual instruments: always include an estimated digit. For example, using the centimeter ruler in Figure 1, the
pencil might be recorded as having a length of 1.87cm. In this number, the “7” is the estimated digit. Another
acceptable reading would be 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.
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: 2.300x10 3 g includes 4 s.f. 2.30x10 3g includes 3 s.f. 2.3 x 10 3 g 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:
Addition and Subtraction: the result is rounded to the same number of decimal places as the least precise number in
the calculation:
: the least precise number is only known to the tenths place, so the answer can only be reported to the tenths
place. 121.0g-4.34g=116.66g⇒round to 116.7g (round to tenths place)
• When you perform a series of calculations, round after each different operation, perform addition and subtraction:
get the answer to the correct number of significant figures. Then perform multiplication and division. It is important
to remember, that you can gain or lose significant digits when you add and/or subtract numbers together!
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’s running ten minutes late, it is not accurate.
VOLUME AND MASS MEASUREMENTS (Refer to the appropriate sections in the in the Laboratory Techniques
handout 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 section of the experiment you will learn to use the buret, 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. Typically, precision is defined as the reproducibility of the results, or how many decimal places you can
measure with a particular device. If your answers are grouped together and you can get the same reading each time,
your results are precise. Accuracy is how close your value is to some “true value”. In some cases we need to use an
average instead of a “true value”. 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 class average when 10 students took 3 measurements each using the same
type of measuring device was 42.12 cm. This person made very precise measurements, but the results were not very
accurate when compared to the class average.
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 measurement.
Beaker volumes are always whole number readings.
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.
Buret
The buret is also designed to accurately deliver a volume of liquid. In other words, you are measuring the amount of
liquid removed from the buret. You will be using a 50 mL buret in lab. The buret has a very small width compared to
length (see Figure 5a and b) allowing for easy reading of the meniscus, and therefore, the volume. The buret volume
can be measured to the hundredths place ( + 0.01 mL). Liquid is drained from the bottom of the buret. Numbering
of the buret, therefore, starts at the top of the buret. Volume readings are made by (1) filling the buret to the 0.00 mL
mark (first reading located at the top of the buret) or some initial volume, (2) drawing off the desired volume of
liquid, (3) measuring the new volume reading on the buret and, (4) subtracting the initial buret reading from the final
reading to obtain the delivered amount of liquid.
Pipets
Pipets (Figure 6) 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. The pipet is designed to deliver a volume of liquid by gravity. Liquid is sucked into the pipet using a pipet
bulb. 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. 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. The 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. Pipets, just like burets, must be read to the hundredths place .
Measurement Procedure:
This section should be completed with a partner – meet someone new and partner up! Obtain two 100 mL beakers.
Fill one of the beakers (called the stock beaker) with ~ 50 mL of water. Set the other beaker aside. Obtain a 10.00 mL
pipet, a 10.0 or 25.0 mL graduated cylinder, and a buret. You will perform the same procedure 4 times, using a
different piece of measuring equipment. Review the glassware section of this laboratory handout to remind
yourself about significant figures and measuring using these pieces of glassware.
Many times it is also necessary to measure the mass of a liquid or solid. You will use an electronic top-loading
balance to perform these measurements. These balances can be read to + 0.01 grams. When you use any balance be
sure to record your data to the appropriate number of significant figures, even if they are zero (20.00 g not 20 g).
Don’t forget – each piece of glassware is used twice – record data in a table in your lab notebook. Use the
format of the table below.
Part 1: The beaker
Mass the dry beaker, record this mass under the starting mass of the beaker for the beaker portion of the experiment.
Using the stock beaker, which you previously filled with ~ 50 mL of water, pour 20-30 mL of that into your preweighed dry beaker.(Note: you do not need to measure exactly twenty milliliters, but you do need to record the exact
volume.) Record the volume of the water in the beaker exactly (with the correct number of significant figures) in the
table below. Mass the beaker and the water. Record the mass of the beaker and water in the data table in the beaker
portion of the experiment. Pour the water out of the beaker, and dry the beaker with a paper towel. Repeat for trial 2.
Part 2: The pipet
Re-mass the dry beaker, record the mass of the beaker in the table below. Pipet 10 mL of water from the stock
beaker. Again, you need not get exactly 10 mL but you should record the actual volume of water you placed in the
pipet with the correct number of significant figures. Drain the water from the pipet into the dry beaker (remember
not to blow out the last little bit!). Mass the beaker with the water and record the mass in your data table. Pour out
the water and dry the beaker with a paper towel. Repeat for trial 2.
Part 3: The graduated cylinder
Re-mass the dry beaker, record the mass of the beaker in the table below. Fill your graduated cylinder with 20-30 mL
of water. Again, you need not get exactly 20 mL of water but you should record the exact volume of water in your
graduated cylinder with the correct number of significant figures (see the pre-lab!). Pour the water from the
graduated cylinder into the dry beaker. Mass the beaker with the water and record the mass in the data table. Repeat
for trial 2.
Part 4: the buret
Re-mass the dry beaker, record the mass of the beaker in the table below. Fill your buret with water. You need not fill
the buret all the way to the 0.00 mL mark, but fill it with a substantial amount of water. Open the stop-cock to drain
out some of the water which will fill the tip of the buret. Close the stop-cock. Record the starting amount of water in
the buret (simply read the location of the meniscus) with appropriate significant figures. Record the starting volume
in the data table. Open the buret to dispense 15-30 mL of water directly into the dried, pre-weighed beaker. Again,
you need not get exactly 15 mL of water but you should record the final volume of water with the correct number of
significant figures. The amount of water dispensed by the buret = final volume measurement – initial volume
measurement. (example: if you filled the buret and read the initial volume at the meniscus it reads 0.55 mL. You
dispense a volume of water – you want it to be around 15 mL. You open and then close the stop-cock. Your final
volume reading of water at the meniscus is 18.12 mL. The total volume dispensed by the buret = 18.12 – 0.55 mL =
17.57 mL)
Calculations: For each step, calculate the mass of the water that you added. Then use the mass of the water and the
volume of the water to compute the density, given that density = mass/volume. Show your calculations in the table
where needed.
Questions:
a. Which volume-measuring device was easiest to use?
b. For each piece of glassware, calculate the average density. Show all your work: (don’t forget units!) **hint:
perform addition first to determine correct number of sig figs
c. 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!
d. Which device was most accurate? Which was most precise? Explain.
DENSITY (adapted from Green River Community College Chemistry Lab)
Purpose
This lab exercise will provide a review of laboratory techniques for measuring the mass and volume of substance.
The proper use of significant figures, metric units and the use of labels in all calculations will be emphasized. Your
job is to come up with a method for measuring the density of various substances and to use proper techniques to
experimentally determine the density of each substance as accurately and precisely as the lab equipment allows. The
ultimate goals of this lab are to determine the densities of two unknown liquids (Activity 1), find the inner volume of
a Beral pipette (Activity 2) and measure the density of and identify an unknown ionic salt (activity 3).
Introduction Pure substances may be characterized by their physical and chemical properties. These properties are
useful in identifying unknown substances. In this lab you will determine the density of different substances (two
unknown liquids and an unknown solid) by measuring the mass and volume of each substance as precisely and
accurately as possible .
For liquids and solids the mass is usually expressed in grams (g) and the volume in milliliters (mL). Note that 1 mL
= 1 cm3 . Thus, in order to calculate the density of an object you need to know both its mass and its volume.
For this experiment, and all future experiments too, all of your experimental data must be recorded in your
laboratory notebook according to the guidelines laid out in the lab report format. You will also be expected to use of
significant digits correctly, to show all calculations, and to label all numbers with the correct units. Keep in mind
that a part of your grades on lab reports will be based on the correct use of the lab notebook .
Before starting the lab devise a method for Activities 1 and 2, below. Before you begin any measurements in lab,
your team must come to a consensus for each procedure, and record an outline of each finalized procedure directly
into your lab. Record each procedure in sufficient depth that a competent AP Chemistry student could replicate the
experiment and get reasonably good results. The procedure should include the type and size of glassware any used.
Density Activity #1. Choose 1 material from the following list: aluminum, brass, copper, oak, pine, polypropylene, PVC,
steel, zinc. Make measurements and record data for 1, 2, 3, & 4 blocks of each material in the data table below. Repeat
the process using marbles. Use the correct number of significant digits in your measurements and calculations.
Substance/# blocks
/1
/2
/3
/4
marbles / 1
marbles / 2
marbles / 3
marbles / 4
Total Volume (cm3 )
Total Mass (g)
Density (g/cm3 )
a. For each material, calculate the average density of the blocks. (Show your calculations)
Substance _____________________
average density ___________________
Substance _____________________
average density ___________________
b. Using Excel, plot the mass vs. volume for each of the materials (you will have 4 points for each material.) For each
material, add a trend line and determine the slope. (Attach print outs of your graphs where you have added a statement
indicating the trend line’s slope with units)
c. How does the slope of the best fit line compare to the calculated average density?
Density Activity #2. What is the total inner volume of a Beral pipette?
Devise and carry out an experiment to determine as precisely as possible (i.e. to 3 decimal places) the total inner
volume of a Beral pipette (figure 1). In addition to a Beral pipette, the only other things you are permitted to use are
distilled water, a beaker, a thermometer, an electronic balance and the table of the density of water at various
temperatures (Table 1 on the next page). Describe the method you developed to solve this problem in enough detail
so a competent AP Chemistry student could repeat the experiment and get reasonable results.
Create data table for activity 2. Number the table as table 2 and give it an informative title. Include the following in
the table: all measurements made in activity 2, all calculated values (including the volume of the pipette!), the
temperature and density of the distilled H2 O used. Use units of measure and correct significant figures for all
measured and calculated values.
Clean-up! When you have finished the lab work
- Rinse out all glassware with tap water, followed by a rinse in distilled water; shake off excess water
- Clean up and dry your work area before leaving lab.
- Clean the balances and the table-top around your assigned balances. Points will be deducted from the lab report
of all team members if your assigned balances and the surrounding table top are not left spotlessly clean.
Questions (show all calculations)
1. A plastic container has a mass of 21.6206 g when clean, dry, and empty. When the container is filled with distilled
water, the total mass is 30.5674 g at a temperature of 20.0 oC. The density of water at this temperature is 0.9982 g/mL.
What is the precise volume capacity of the plastic container?
2. The plastic container in question 1, above, is filled with an unknown liquid and then weighed. The total mass is 27.6916
g. What is the density of the unknown liquid?
3. Suppose you have a small cube of ligna vitae, a very dense tropical wood. If it measures 1.5 cm on a side and has a
mass of 4.00 g, could it float in the unknown liquid from in question 2, above? Explain and support your response with a
calculation.
4. Using the following data to graphically find the density of a substance. Graph the data in Excel. Add a trend line and
determine the slope of this line. Make sure that you label the axes and title the graph appropriately and attach a printout of
the graph.
Remember you are finding the density of the substance only.