Volumetric Flask
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A Description
A volumetric flask is used to measure very precisely
one specific volume of liquid (100 ml, 250 ml, etc., depending
on which flask you use). This flask is used to prepare a solution
of known concentration. A volumetric flask should not be used to heat substances or store solutions, and you should avoid pipetting directly from the volumetric flask.
Equipment typically used when preparing a solution in a volumetric
flask
includes the volumetric flask and its glass stopper, a medicine dropper,
and a funnel.
Water can be delivered from the beaker or wash bottle.
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Why do you think a medicine dropper is used when preparing
solutions in a volumetric flask? Answer
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Preparation of Glassware
Beads of water formed on flask walls...
Clean the volumetric flask before use. Make sure that beads
of distilled water do not adhere to the inside walls of the flask.
Check especially the region between the volumetric mark and the
mouth of the flask.
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What is the best way to eliminate water clinging
to the walls of the flask?
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A. Dry the neck with cheesecloth.
B. Dry the neck with a paper towel.
C. Wash the flask with laboratory soap and rinse it thoroughly.
D. Add distilled water to the flask until the beads are washed down.
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Answer
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Dissolving a Solute
When dissolving a solid or a liquid, remember to add the chemical
to water, rather than vice versa, especially when diluting concentrated
acids.
For solutions of solid in a liquid solvent, first dissolve the solid
in a vessel other than the volumetric flask, in case the solid
must be heated or crushed in order to be dissolved. After the solid dissolves, pour the liquid into the volumetric flask and use the solvent to rinse all of the solution from the vessel into the flask. However,
if the solid is known to be very soluble, it may be added directly
to the flask.
Video: Dissolving solid in flask ( 2.85 M )
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Video ( 5.07 M ) |
The student in the video believes that he prepared a 0.5236 M NaCl solution. Is the actual molarity higher or lower than 0.5236 M? Why? Answer |
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Preparing and Storing a Solution
To make the solution in the volumetric flask, add the liquid solvent
to the flask until the solution level is just below the neck of the
flask. Stopper the flask, and mix by turning upside down and back.
After thorough mixing, slowly add solvent until the meniscus is aligned
with the mark when viewed directly at eye level:
Video: Filling the volumetric flask ( 7.39 M )
Transfer the solution to a storage bottle. Once prepared,
a solution should almost never be stored in a volumetric flask.
Video: Transfer from volumetric flask
to storage bottle ( 2.26 M )
Finer Points
Mix the solution well when the level of liquid is near the
bottom of the neck of the flask.
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| Before
mixing | After mixing |
Video: Mix Before Aligning Meniscus ( 1.39 M )
Touch the outside of the flask and verify that your solution is at approximately room temperature before final
dilution to the mark. Why?
Use a medicine dropper to bring the solution to the mark.
Video Final meniscus alignment ( 1.46 M )
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Video ( 990K ) |
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What should you do to correct this error?
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A. Remove excess liquid with an eyedropper
B. Prepare the solution again
C. Add a little more solute
D. Use the solution as it is; the concentration is close enough to what you wanted.
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Answer |
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Use a stopper when mixing the solution!
Video: "Using your hand as a stopper" ( 1.05 M )
Miscellaneous Exercises
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Video ( 2.35 M ) |
What is wrong with the procedure shown in the video? Answer |
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Video ( 3.30 M ) |
How could the student have avoided this problem? Answer |
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