Intro to Chemistry Chem1020 Lab
Identification of
an Unknown Liquid
Chemistry Department
Minneapolis Community & Technical College
1
Overview
• Part I
Introduction
• Part II
Solubility determination
• Part III
Density measurement
• Part IV
Boiling point determination
• Part V
Identification
2
Part I. Introduction
In this experiment, you will be given a pure unknown
liquid and the goal is to determine which one of the
eight known substances this mysterious liquid is most
likely to be.
In order, you will determine its three physical properties
(meaning that you are not changing its chemical
composition throughout the experiment). They are:
• solubility in water and ethyl alcohol,
• density, and
• boiling point.
Make sure to record
Then you will compare these properties with the
properties of those of eight candidates. The best match
will suggest the identify of your unknown liquid.
this unknown number
on your data sheet.
Caution: All procedures must be performed in the fume hood. Make sure
to cover the containers whenever they are moved out of the fume hood.
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Part II. Solubility
1. Your instructor will first demonstrate that when two liquids are mixed,
how will it look like when they are “soluble” or “insoluble”.
2. Then you will determine the solubilities of your unknown liquid in
water and ethyl alcohol, respectively.
Test tube 1:
Test tube 2:
20 drops of unknown
+
20 drops of water
20 drops of unknown
+
20 drops of ethyl alcohol
Soluble or Insoluble
Soluble or Insoluble
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Precautions!!!
1. Use the medicine dropper
on your bench for the
unknown liquid only.
2. Do not let the tip of any
dropper touch the test tube,
especially the inside. Put the
dropper back to its original
container after each use.
3. Before recording the
observation, mix the two
liquids by gently flicking the
bottom of the test tube.
4. Dispose the wastes from
both tubes into the
designated waste container.
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Part III. Density
• You will use the same method as you did in the previous
“Measurement” lab to determine the density of this unknown
liquid.
• Instead of a volumetric pipette, a 10-mL graduated cylinder
will be used to accurately determine the volume of ~5 mL
liquid.
• Record the mass measurements as XX.XX g.
• Record the volume measurements as X.XX mL.
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1.
2.
3.
1. Determine and record
the mass of an empty
10-mL graduated
cylinder.
2. Use the medicine
dropper to transfer
approximately 5 mL of
liquid into the cylinder.
5.
4.
3. Determine and record
the volume of the
liquid. (Often being
forgotten)
4. Determine and record
the total mass of
cylinder and the liquid.
5. Pour the liquid into the
large test tube on your
bench for later boiling
point measurement.
6. Repeat steps 2-5 for a
second trial.
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Sample Calculation of Density
Trial 1
Trial 2
Vunknown
5.12 mL
4.99 mL
Mcylinder +unknown
40.71 g
40.56 g
Mcylinder
36.66 g
36.66 g
Munknown
= Mcylinder + unknown - Mcylinder
40.71g – 36.66g
= 4.05 g
(2 decimal places)
40.56g – 36.66g
= 3.90 g
(2 decimal places)
3.90g / 4.99mL
Dunknown = Munknown / Vunknown 4.05g / 5.12mL
= 0.791 g/mL
= 0.782 g/mL
Daverage = (D1 + D2 )/2 = (0.791 (3
+ sig.
0.782)/2
=
0.787
g/mL
fig.)
(3 sig. fig.)
• Since the difference between these two density determinations
(0.791 – 0.782) = 0.009 g/mL is small enough (< 0.03 g/mL),
a third trial is not necessary.
• Follow the rules of significant figure calculation strictly.
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Part IV. Boiling Point
1. Drop a piece of white
rock, called boiling
chip, into the large test
tube. The chip ensures
a smooth boiling and
prevents “bumping”.
3. Lower the test
tube into a water
bath sitting on a
hot plate. Make
sure all the
unknown liquid
is submerged in
the water bath.
1 cm
2. Insert a split-hole stopper
with a thermometer into
the test tube which should
have ~10 mL of the
unknown liquid.
4. Turn the knob on
the right to dial
“6” and start
heating.
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Part IV. Boiling Point
• Watch the unknown liquid inside the large test tube closely. When there
are abundant strings of bubbles arise from the boiling chip, read the
thermometer immediately. Record this temperature as the boiling point
in the form of XX.X ºC. Waiting too long will boil off too much liquid
resulting in inaccurate determination.
• Raise the large test tube so that it is out of the water bath.
(Often forgotten)
• Once cooled, dispose of the remaining unknown liquid and
the boiling chip into the designated waste container.
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Part V. Identification
1. Fill in the table on p.4 of
the protocol with data of
your unknown liquid. See
the example shown in blue.
0.787
68.1
soluble
soluble
2. First cross out unlikely
candidates based on
solubility.
3. The remaining four
candidates unfortunately
all have density around
0.79 g/mL, so we have to
look for the one with a
boiling point closest to
68.1 ºC , that of our
unknown. It looks like methyl alcohol has the closest one. We got a winner!
Acetone has the second closest boiling point, so can be put down as the 2nd most
likely candidate.
If our unknown had a boiling point of 61.0 ºC, then methyl alcohol and acetone
would be equally possible. No conclusive determination can be made.
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• Put all of the used glassware into the
designated bins for later cleaning. Put the
remaining unknown liquid at a designated
place.
• Wipe your station with wet sponge.
• Please restock your station with clean
glassware.
Used small test tubes and medicine droppers
Used large test tubes and graduated cylinders
Vials with remaining unknowns
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