Name:
AP Chemistry: Laboratory Techniques
Objective
Familiarity with the various instruments used for making physical measurements in the laboratory is essential to
the study of experimental chemistry. In this experiment, you will investigate the uses and limits of the various
types of laboratory balances and volumetric glassware.
Part I: The Laboratory Balance and Mass Determinations
Mass is a direct measure of the amount of matter in a sample of substance, that is, a direct indication of the
number of atoms or molecules the sample contains. There are various types of balances available in the typical
chemistry laboratory. Such balances differ in their construction, appearance, operation, and in the level of
precision they permit in mass determinations. Simple rules to follow when using a balance:
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Never put chemicals directly on the pan – always use a weigh boat, weigh paper or glass vessel.
Immediately clean up spills on the metal or scale
Never put hot objects on the balance
Always make sure that the balance gives a reading of 0 grams when nothing is present on the pan.
Adjust the tare / zero knob if necessary.
Balances can be damaged by moisture. Be careful with liquids used around the scale.
The digital electronic balance gives the mass directly when an object is placed on the pan.
This balance reads to 2 decimal places,
and has a 200-gram capacity.
This balance reads to 1 decimal place,
and has a 400-gram capacity.
Procedures in a laboratory exercise are generally written in such terms as “weigh 0.5 grams of substance…to the
nearest milligram.” This does not mean that exactly 0.500 gram of substance is needed. Rather, the statement
means to obtain an amount between 0.450 and 0.550 gram. Unless a procedure states explicitly to weigh out an
exact amount, you should not waste time trying to obtain an exact amount. However, always record the amount
actually taken to the precision of the balance used.
For accurate mass determinations, the object to be weighed must be at room temperature. If a hot or warm
object is placed on the pan of the balance, the air around it becomes heated. Warm air rises, and the motion of
the air might be detected by the balance, giving masses that are significantly less than the true value.
Procedure 1: Using Laboratory Balances
Materials: small rubber stopper, large rubber stopper, small beaker, balance, weigh paper
Directions: Examine the different types of balances provided in the laboratory. Determine the mass of a
small beaker that can accommodate the objects whose masses are to be determined. Transfer one of the
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objects to the beaker. Determine and record the combined mass; determine the mass of the object by the
subtraction method. Then determine and record the mass of the object by itself in the accepted value
section. Repeat this procedure with the other balances. (The average mass will be the accepted value)
Mass of
beaker
Mass of
beaker +
stopper
DATA TABLE: Massing the small stopper
Mass of stopper
Mass of
(by subtraction)
stopper
Show work
(by weighing)
(accepted
value)
% error
(show work)
Electronic
balance
(hundredths)
Electronic
balance
(tenths)
Mass of
large
rubber
stopper
Mass of
beaker +
stopper
DATA TABLE: Massing the large stopper
Mass of stopper
Mass of stopper
(by subtraction)
(by weighing)
Show work
(accepted value)
% error (show work)
Electronic
balance
(hundredths)
Electronic
balance
(tenths)
Part II: The Use of Volumetric Glassware
This experiment makes use of several types of volumetric glassware, specialized pieces of
glassware which are used to measure volumes of liquids very precisely in quantitative
laboratory work. Examples of these types of glassware include graduated cylinders, burettes,
pipets, and volumetric flasks. These are carefully manufactured and expensive items and
should be treated with great care. Ordinary beakers and flasks are not used for accurate
volume measurements; the markings on these pieces of glassware are generally only
accurate to within 5%
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Volumetric glassware is usually marked with its total volume, the notation “TD” or “TC”, and a temperature
(usually 20°C). The marked temperature indicates the temperature at which the apparatus was calibrated. Since
density and volume change with temperature, the volume markings are strictly correct only at the calibration
temperature. The notations TD and TC stand for the phrases "to deliver" and "to contain." The TD notation, used
on burettes and pipets (and some graduated cylinders), means that the apparatus is calibrated to accurately
deliver or transfer the stated volume to another container. The TC notation, used on volumetric flasks and (most)
graduated cylinders, means that the markings give an accurate measure of the volume contained, but that
pouring the liquid into another container will not necessarily deliver the indicated volume.
Procedure 2: The Use of Volumetric Glassware
A. Graduated Cylinders
The most common apparatus for routine determination of liquid volumes is the
graduated cylinder. Although this instrument does not permit as precise a
determination of volume as do other devices, it is sufficient for use in the lab. A plastic
safety ring is often used to protect the cylinder if it is knocked over.
1. Determine the smallest graduation of volume that can be measured with each
cylinder. You should read the instrument to one more decimal place to the
right than the smallest graduation mark. This last number will be an
estimation on your part.
2. Read the bottom of the meniscus for water and aqueous solutions.
3. Greater precision is possible with smaller cylinders and more divisions.
4. Read at eye level.
5. What do the following graduated cylinders read? A._______ B._________ C. ________
Answers:
A: Each of the smaller lines represents 1 mL. The reading will be one place to the right of the decimal. The
bottom of the curve is about halfway between the 16 and the 17, so the reading on this graduated cylinder would
be 16.5 mL.
B: Each line represents 0.1 mL. The reading will be two places to the right of the decimal. The bottom of the
curve looks like it is exactly on the line that would represent 3.8. Because it is exactly on the line, the reading
would be 3.80 mL.
C: The bottom number is 6 and the top number is 8. If each mark is 0.2, the measurement is halfway between 7.4
and 7.6 – which is 7.5. Because each line is equal to 0.2, the measurement will be 2 places past the decimal. The
correct way to report this measurement is 7.50 mL.
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Directions: For each graduated cylinder, determine the smallest graduation mark. Use that graduation mark to
correctly measure the volume of the liquid.
Data Table: Graduated Cylinders
Water Color
GREEN
Maximum value of
graduated cylinder.
Smallest graduation
mark
Volume of liquid
BLUE
RED
B. Pipettes
When a more precise determination of liquid volume is
needed than can be provided by a graduated cylinder, a
transfer pipette may be used. The Mohr pipette is
graduated so as to allow the dispensing of varied volumes.
The volumetric transfer pipette is made is dispense one
fixed volume with high accuracy. Small plastic pipettes may
or may not be calibrated; it might be necessary to count the
drops to determine the volume of one milliliter. When using
a pipette, a pipette filler or a rubber bulb can supply the
suction needed to fill the pipette.
 Never pipette my mouth.
 Roll the knob up on the pipette to apply
suction force. Allow the liquid to be drawn above the calibration mark
on the barrel. At this point, release liquid until it reaches the mark.
Then, quickly place the pipette filler over a vessel and dispense the
measured liquid.
 Touch the tip of the pipette to the side of the flask to remove the last
drop of liquid.
Materials: 50mL beaker, plastic pipette, graduated pipette, volumetric pipette, pipette filler, thermometer
Directions:
1. Ensure that the graduated and volumetric pipette is clean. Rinse with water. An ultra-clean pipette will
not bead up anywhere inside the barrel.
2. If measuring a substance other than water, to prevent dilution of the solution, always rinse the pipette
with several small portions of the solution.
3. Practice using each pipette until you feel comfortable with the technique.
4. Starting with the graduated pipette, fill with water close to but under the zero mark. Record the pipette
reading in the data table with the proper number of digits (look at the graduation marks!) under pipette
volume.
5. Using your balance, tare (zero) a clean, dry 50mL beaker.
6. Empty the contents into the beaker. Record the mass of the water.
7. Obtain the temperature of the water. Record.
8. Using the Density of water table, record the density of the water at that temperature
9. Calculate the volume using the density and mass.
10. Compare the volume to the volume of the pipet as specified by the manufacture. Any significant
difference between these two volumes in an indication that you need additional practice in pipetting.
11. Calculate percent error – assume the calculated volume is the actual volume.
12. Repeat the procedure with the remaining pipettes.
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DATA TABLE: PIPETTES
Pipette
Initial
reading
Volume
of water
dispensed
Mass
of
water
Water
temp
Water
Density
Calculated volume
(show work)
% Error
(Show work)
Calculated volume
(show work)
% Error
(Show work)
(10–initial)
Graduated
pipette
Pipette volume
Mass
of
water
Water
temp
Water
Density
volumetric
pipette
Plastic
pipette
C. Burettes
When samples of various sizes must be dispensed or measured precisely, a buret
may be used. The buret consists of a tall, narrow calibrated glass tube, fitted at
the bottom with a valve (called a stopcock) for controlling the flow of the liquid.
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Buret must be scrupulously clean before use; precision permitted is
0.02mL, but if not completely clean, this level of precision is not attainable.
Rinse with tap water, followed by distilled water.
Buret does not have to be filled to the 0.00 mark; fill to a level that is
comfortable using a small buret funnel.
Readings should be made to the 0.02mL; volume of liquid dispensed is
obtained by subtraction of the two volume readings.
The glass tip of the buret should touch the flask or beaker so that the last
drop is accounted for.
Materials: Burette, Burette Clamp, Ring stand, Flask, 100mL beaker, thermometer
Directions:
1. Obtain a burette and set it up in a clamp on your lab bench as shown.
2. Ensure that the burette is clean using the information given above.
3. Fill the burette to above the zero mark. Examine the burette while the water is running from it. If the
burette is clean enough for use, water will flow down the inside of the burette without beading up
anywhere. If the burette is not clean, clean again or replace.
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4. Refill the burette with distilled water to a point somewhere below the zero mark. Record the precise
level to the nearest 0.02 mL.
5. Obtain a clean and dry 100mL beaker. Mass the beaker.
6. Place the weighed beaker under the burette. Open the stopcock of the burette and run water into the
beaker until approximately 25 mL of water has been dispensed. Determine the precise liquid level in the
burette to the nearest 0.02mL and record.
7. Mass the beaker and the water dispensed.
8. Calculate the volume of water that has been dispensed from the burette by subtraction.
9. Take the temperature of the water. Record
10. Determine the density of the water using the chart provided.
11. Calculate the percent error (assuming the calculated volume is the actual volume)
BURETTE DATA TABLE
Initial Volume Reading
Mass of beaker
Final Volume reading
Mass of beaker and water
Volume of water dispensed
(show work)
Mass of water dispensed.
(show work)
Temperature of the water
Density of the water
Calculated volume of the water
(show work)
Percent Error
(show work)
D. Funnels
Filtration is performed by inserting a paper filter into a glass (or plastic) funnel. Write your period and group
number on the filter paper. Weigh and record the mass of the filter paper. The paper must be folded in half twice
to form a cone, which is then inserted into the funnel. The paper filter is then moistened with distilled water
using a wash bottle.
Materials:
Beaker, Buchner funnel, filtering flask, stir rod, filter paper, sand, rubber spatula, wash bottle
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Directions:
1. Weigh and record the value of approximately 5 grams of sand.
2. Make a mixture by adding approximately 25 mL of tap water to the 5 grams of sand into a beaker
3. Mix with a glass stirring rod, and while the mixture is in motion, filter the sand as demonstrated by the
teacher. Be sure to remove all the sand from the beaker with the wash bottle.
4. Allow the filter paper (with the sand) to dry overnight.
5. Mass the sand and calculate the percent error.
DATA TABLE FUNNEL
Mass of Sand before
filtration
Mass of Sand after
filtration
Percent error
(show work)
LABORATORY QUESTION
1. Pipets used for the transfer of samples of solutions are always rinsed with a small portion of the solution
before the actual sample is taken. Calculate the percent error likely to arise in an experiment if 1-mL, 5mL, and 10-mL pipets are used for transfer and each pipet contains 5 drops of water adhering to the
inside of the barrel. A single drop of water has a volume of approximately 0.05 mL. Show all work; box
answers.
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