Rasmussen College Transferring Solids and Liquids Lab Report

Laboratory TechniquesLaboratory Techniques
This laboratory session is designed to help you become familiar with various measuring devices
used in the laboratory, while learning proper laboratory techniques. Success in the laboratory
will depend upon your ability to carefully measure, record and follow proper laboratory
techniques. While working in small groups, make sure everyone performs each of the following
laboratory techniques and measurements. This lab is also designed to help you feel comfortable
in the lab, so relax, take your time and enjoy your experience of working in the lab.
OBJECTIVES
In this experiment, you will
• Become familiar with various laboratory techniques.
• Learn how to record the proper number of significant figures for any measurement.
MATERIALS
1 mg electronic balances
Screw top jar with NaCl
10 mL Mohr pipet
Pipet bulb
25 mL volumetric pipet
Filter paper
Buchner funnel with flask
Boric acid
Copper sulfate pentahydrate
charcoal
PROCEDURE
Transferring Solids and Liquids
Chemical containers have either screw-top lids or glass stoppers. When removing a screw-top lid
be sure to set it down on the bench, top-side down. Glass stoppers should be held during the
transfer of chemicals. Remember to replace the lid after you are done. Careful handling of lids
and stoppers will help to prevent contamination.
Never return unused reagents to their containers. Excess reagents should be placed in the
appropriate waste containers.
All spills should be cleaned up immediately. Report all spills, no matter how small, to your
instructor.
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Laboratory Techniques
1. Fill a 250 mL beaker with tap water, and while using a stirring rod decant (pour water down
stirring rod) approximately 100 mL of the water to a 250 mL Erlenmeyer flask. Repeat this
process two more times.
2. Using the glass jar filled with sodium chloride transfer a small portion of sodium chloride to
a 100 mL beaker by tapping or rolling the glass jar. Return the sodium chloride to its
container and repeat two more times.
Using the electronic balance
1. Using a 0.001 g balance (milligram balance) measure out approximately 10 g of sodium
chloride into a 50 mL beaker and record its mass in the data table. Do not pour the sodium
chloride into the beaker while it is on the balance.
2. For comparison purposes, re-weigh the 50 mL beaker and NaCl sample from step 1 above
using a different 0.001 g balance. Record the value in the data table. Are the measurements
exactly the same? Should you use the same balance between measurements and throughout
an experiment?
3. Place the sodium chloride back into the original container.
Reading Scales
Correctly reading a scale is a skill that is important to master. The pipet, buret, graduated
cylinder, and many other instruments and devices utlilize scales that must be read properly for
successful laboratory work.
Determining the Increment
A scale is made up of a series of graduations. Usually, some of the graduations are labeled at
regular intervals, with smaller, unlabelled graduations between them. The increment between the
graduations will dictate how many significant digits should be recorded from the scale. Any data
recorded from a scale should include all of the certain digits and one digit that is estimated by the
person reading the scale.
To read the scale, you must first determine the scale increment. The scale increment is the
quantity between any two adjacent graduations. To find the scale increment, subtract the values
of any two adjacent labeled graduations and divide by the number of intervals between them.
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Laboratory Techniques
Estimating the Uncertain Digit
The uncertain digit in any measurement is estimated using an imaginary linear scale between the
two smallest graduation marks of the measuring device.
In the example below, the length of the solid rectangle is between 2.5 and 2.6 cm. However, the
rectangle is clearly not 2.50 or 2.60 cm, and therefore a more accurate length could be recorded.
An imaginary linear scale between the two graduation marks can be used to make a more
accurate determination of the length. The edge of the rectangle is approximately 8/10 of the way
between the 2.5 and 2.6 graduation marks. So, the uncertain digit is an “8”, and the length to be
recorded for the rectangle is 2.58 ± 0.01 cm. The use of the “±” symbol indicates the uncertainty
in the last digit. Any recorded measured value is always understood to have a ± 10% uncertainty
in the last recorded digit. All recorded measured values contain certain digits plus one uncertain
digit (with a 10% uncertainty).
Sample problems
1. Using the displayed ruler, how long is the solid in the image below? Record in data table. Make
sure you record to the correct number of decimal places.
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Laboratory Techniques
2. Using the displayed ruler, how long is the solid in the image below? Record in data table. Make
sure you record to the correct number of decimal places.
Measuring volumes
Graduated Cylinders
Graduated cylinders are used to measure the volume of liquid samples and are available in many
different sizes. The measurement accuracy of a graduated cylinder is rather poor (may be as
much as 10% off) so you must consider the desired accuracy before choosing to use a graduated
cylinder. Generally, graduated cylinders are not appropriate for the preparation of analytical
solutions – pipets, volumetric flasks, and burets should be used instead.
APPROXIMATE VOLUMES – An approximate volume of a liquid can be delivered by filling
the graduated cylinder to the desired level and transferring the entire contents of the cylinder to a
receiving vessel.
MORE ACCURATE VOLUMES – More accurate volumes are delivered by making an initial
and final volume reading and calculating the volume delivered (by difference). An accurate final
volume reading can only be made if there is liquid still in the graduated cylinder; thus, the
graduated cylinder should not be emptied when the contents are transferred.
Determine the volume contained in a graduated cylinder by reading the bottom of the meniscus
at eye level. Read the volume using all certain digits and one uncertain digit. Certain digits are
determined from the calibration marks on the cylinder. The uncertain digit (the last digit of the
reading) is estimated.
Reading the volume from a 100 mL graduated cyclinder
Step 1: Determine the scale increment: To find the scale increment, subtract the values of any
two adjacent labeled graduations and divide by the number of intervals between them.
Step 2: Use the graduations to find all certain digits: Use the labeled graduations and the scale
increment to find the certain digits in the measurement.
Step 3: Estimate the uncertain digit and obtain a reading: Estimate the distance that the
meniscus lies between the two graduations as a decimal fraction and multiply by the scale
increment.
1. Fill a 100 mL graduated cylinder approximately half full with tap water and record the exact
volume of water in data table. Make sure you record to the correct number of decimal places.
2. Transfer approximately 10 mL of tap water to a 250 mL beaker. Record the new volume of
water in the graduated cylinder. Calculate the volume of tap water transferred to the beaker.
Record the transferred value in the data table.
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Laboratory Techniques
Mohr pipet
A Mohr Pipet is a graduated pipet that is designed to deliver small portions of a liquid or
solution. These portions are determined by recording the difference between the initial and final
volume readings. Thus, it is important to note the total capacity of the Mohr pipet, the scale
increment, and whether the pipet is to deliver or to contain.
1. Obtain a 10 mL Mohr pipet (pipet with 100 small increments) and pipetting bulb.
2. Fill a 250 mL beaker half full of distilled water.
3. Record the mass a 100 mL beaker.
4. Using the Mohr pipet transfer approximately 7 mL of distilled water from the 250 mL beaker
into the 100 mL beaker. Record the initial and final volume readings of the Mohr pipet.
5. Using the initial and final volume readings of the Mohr pipet calculate the exact volume of
water transferred to the 100 mL beaker.
6. Record the mass of the 100 mL beaker plus the added distilled water from step 4 above.
7. Using the mass of the 100 mL beaker and the mass of the 100 mL beaker with added distilled
water, calculate the exact mass of water transferred to the 100 mL beaker.
8. Using the calculated mass and volume measurements determine the density of the distilled water.
Volumetric Pipet
Pipetting involves drawing a liquid into a pipet and allowing liquid to drain from the pipet in a
controlled manner. Pipetting is used to quantitatively transfer a liquid from one container to
another.
The volumetric pipet has a single graduation that allows it to deliver one specific volume
accurately. There are many different sizes of volumetric pipets (1-, 5-, 10-, 25-, 50-, and 100-mL
volumetric pipets are common). A volumetric pipet is accurate at the temperature at which it has
been calibrated. The temperature where the volume is accurate is usually printed on the neck of
the pipet. If you are working at a different temperature, the volumetric pipet should be calibrated.
1. Obtain a 25 mL volumetric pipet and pipetting bulb.
2. Fill a 250 mL beaker half full of distilled water.
3. Record the mass of a 100 mL beaker.
4. Using the volumetric pipet transfer 25 mL of distilled water into the 100 mL beaker.
5. Record the new mass of the 100 mL beaker plus 25 mL of distilled water.
6. Using the mass and volume measurements determine the density of the distilled water.
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Laboratory Techniques
Heating Materials
Gas Burner
A gas burner is used to heat non-flammable objects or solutions. It can be used to heat objects to
very high temperatures. Temperatures in the hottest region of the burner approach 1900°C.
A gas burner can be used to:
•
heat aqueous non-flammable solutions
•
fire-polish broken glass
•
dry hydrated salts
•
melt samples
•
heat salts to observe emission spectra
Methane (CH4) gas is fed into the burner through the gas inlet. The gas control needle valve
controls the rate at which methane enters the burner. The rate at which air enters the burner is
adjusted with the air control vent. Methane and oxygen mix in the burner tube and, when ignited,
produce a flame.
Gas burners are a very practical and useful tool in the chemistry laboratory. However, they can
be very dangerous if not operated properly. When using a gas burner, it is necessary to take the
following safety precautions:
•
Wear safety goggles or safety glasses. Be certain no flammable materials are present (in the
vicinity around the burner) — papers, textbooks, or other personal items. An open container
of a flammable solvent (ethanol, ethers, or other organic solvents) creates a fire hazard when
operating a gas burner.
•
Protect yourself. Be sure your face, clothing and hair are not above or near the opening of the
burner tube. Avoid loose clothing and tie back long hair.
•
Be careful with hot objects. Burns can occur from objects that appear cool. After heating an
object, allow plenty of time for the object to cool before touching it.
•
NEVER attempt to ignite a burner when you can smell the odor of the gas (the mercaptan
added to the gas).
•
NEVER leave a gas burner unattended. Always extinguish a gas burner before leaving the
area.
1. Practice lighting the gas burner.
Separating Materials
Filtration
Filtration is a technique used to separate a solid from a liquid. The solid is separated from the
liquid phase by passing the mixture over a filtering media. Filtering media is characterized by
being chemically inert to the mixture, and having small pathways for the liquid to pass through,
but these pathways should be smaller than the solid particle size. Filtering media can be prepared
using paper, fritted-glass, or any porous material.
The mixture can be forced through the filter by either gravity or reduced pressure on one side of
the filter (by creating a vacuum). It is possible to separate a solid from a liquid by either
technique, however there are advantages to each technique.
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Laboratory Techniques
Gravity filtration is recommended when:
•
the mixture is hot (above room temperature)
•
the liquid is saturated with one or more reagents
•
the solvent is very volatile (chloroform, alcohols, ethers).
Vacuum filtration is recommended when:
•
the conditions where gravity filtration is recommended are NOT present
•
a quick separation is required
Gravity Filtration
1. Setup and ice bath by placing ice cubes in a 400 mL beaker with tap water. Place a wash
bottle with distilled water in the ice bath for use later in the procedure.
2. Using three different beakers measure out 2 g of boric acid in one, 2 g of copper sulfate in
another and 0.1 g of charcoal in another.
3. Make note of the color of each material in the data table.
4. Setup a gravity filtering system using a 250 mL beaker for the receiver. Recall from the
laboratory techniques video how to fold fluted filter paper for gravity filtration.
5. Place all three substances into a 100 mL beaker. Add 20 mL of distilled water and stir the
mixture with a stirring rod while heating the beaker on a hot plate.
6. Heat the mixture until it just starts to boil. Use a beaker tongs to hold the beaker, and pour its
contents onto the filter paper. Collect the filtrate (the liquid that passed through the filter paper).
7. Describe the color of the solid on the filter paper.
8. Cool the filtrate in ice water for about 10 minutes to allow the boric acid to crystallize. Be
sure not to allow the beaker to tip in the ice bath. A white cloudy solution should be noted.
Vacuum Filtration
1. Use vacuum filtration to collect and separate the white boric acid crystals from the copper
sulfate solution. Do not wash the boric acid crystals with room temperature distilled water as
they will re-dissolve and move through the filter paper. If needed, wash the crystals with only
the minimum amount of ice-cold distilled water. The crystals can be dried by leaving them
under vacuum for 3-4 minutes. Record the color of the crystals in the filter paper.
2. Transfer the filtrate to an evaporation dish. Place the evaporating dish in the fumehood on top
of a piece of paper with your names on it.
3. Allow to dry for one week.
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Laboratory Techniques
Date ___________________
Name ______________________________
DATA TABLE
Using the Electronic Balance
Balance 1
Mass of 50 mL beaker
g
Mass of 50 mL beaker + NaCl
g
Mass of NaCl
g
Balance 2
Balance 3
Omit
Omit
g
Omit
g
Omit
Reading Scales
1. Length of rectangle 1
cm
2. Length of rectangle 2
cm
Measuring Volumes – 100 mL graduated cyclinder
1.
To what decimal place should all measurements be recorded when using a 100 mL
graduated cylinder?
mL
Initial volume
mL
Final volume
mL
Volume transferred
mL
Measuring Volumes – 10 mL Mohr pipet
Temperature of water
°C
Mass of 100 mL beaker
g
Initial volume of Mohr pipet
mL
Final volume of Mohr pipet
mL
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Laboratory Techniques
Volume of water delivered with Mohr pipet
mL
Mass of 100 mL beaker plus distilled water
g
Mass of water delivered with Mohr pipet
g
Density of Water = mass of water / volume of water
g/mL
Measuring Volumes – 25 mL volumetric pipet
Temperature of water
°C
Mass of 100 mL beaker
g
Mass of 100 mL beaker plus distilled water
g
Mass of distilled water
g
Volume of water delivered by 25 mL pipet
25.00 mL
Density of Water = mass of water / volume of water
Separating Materials
1.
Color of boric acid
2.
Color of copper sulfate
3.
Color of charcoal
4.
Color of solid collected in first filtration
5.
Color of solid collected in second filtration
6.
Color of liquid remaining in evaporating dish
16
g/mL