Transcript Observations of Substances

Intro to Chemistry Chem1020 Lab
Measurements
Chemistry Department
Minneapolis Community & Technical College
1
Overview
Making measurements is a very important part in a chemistry lab. In
this lab, you will learn how to make the measurements CORRECTLY.
2. A ruler for length measurement
1. A thermometer for
temperature measurement
3. A graduated cylinder for
volume measurement
4. A balance for
mass measurement
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Part I: Temperature Measurements
Example: 1.
increment
22
21
Always determine the increment of the thermometer first.
For example, the space between 20 and 30ºC in the
thermometer on the left is divided into ten equal spaces.
So the space between any two closest lines, or the
increment is: (30ºC - 20ºC) / 10 = 1ºC (at ones place)
It means that the top of the green liquid inside the
thermometer falls somewhere in between 21 ºC and 22 ºC
even though the numbers 21 and 22 are not really marked
on the thermometer.
2. Now imagine the space between 21 and 22 is divided into
10 even smaller but equal spaces, thus each space will
have a magnitude of (22ºC - 21ºC ) / 10 = 0.1 ºC. With
this assumption, the top of the green liquid may be
around 21.9 ºC. The number 9 on the tenth place is just
an estimation, therefore not accurate. It may also be
estimated as 21.8 ºC or 22.0 ºC. So the final measurement
should be reported as 21.9 ± 0.1 ºC. The 0.1 ºC is called
the uncertainty or error. Note that the uncertainty is
always one digit beyond the increment digit.
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Part I: Temperature Measurements
Using the same approach as described in the previous slide, you will
be measuring the temperatures of cold tap water, boiling water, and
ice-cold water. Since you will be using the same thermometer, the
increment will still be 1 ºC, and the uncertainty or error will be 0.1 ºC.
Hold the thermometer so that
the bottom bulb is in the middle
of the water body.
When the green liquid inside
stops moving, read the
temperature.
Never let the bulb touch the
sides of the beaker, which
may cause inaccurate reading.
x
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Part II. Length Measurements
• Ask a labmate to stand against this pillar in
the lab and mark his/her height (only on the
white paper).
• Use two different meter sticks (Tools 1 & 2)
to measure that height. Both sticks are one
meter long, however, they are graduated
differently. In other words, they have
different increments.
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Part II. Length Measurements
• As with the thermometer, determine the increments of these two
meter sticks first. Just a reminder, the increment is the magnitude
between any two closest lines.
• Always estimate one digit beyond the increment digit.
• For example, if the increment is 0.1 m, the uncertainty/error would be
0.01 m, and the measurement should be reported as X.XX ± 0.01 m.
If the increment is 0.01 m, the uncertainty/error would be 0.001 m,
and the measurement should be reported as X.XXX ± 0.001 m.
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Part III. Volume Measurements
A graduated cylinder is one of the most
common devices to measure the
volumes of liquids in a chemistry lab.
They come in different sizes, and most
likely, different increments as well.
1. First determine the
increments of one 10-mL,
one 25-mL, one 100-mL,
and one 1000-mL
graduated cylinders. You
may find the 25-mL on
your own bench.
25-mL
10-mL 100-mL
1000-mL
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Part III. Volume Measurements
2. Determine the volumes of water in the 10-mL and the 100-mL
graduated cylinders.
Example:
• The increment of this 10-mL
graduated cylinder is (9-8) /
10 = 0.1 mL, so the
uncertainty /error would be
0.01 mL.
• Make sure your eyes are at
the same level as the bottom
of the meniscus.
8.2
8.1
• Estimate where the bottom of
the meniscus is located
between 8.1 and 8.2 mL. The
measurement can be reported
as 8.15 ± 0.01 mL.
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Part IV. Mass Measurements
1. Always use the same balance during
one experiment (or follow
instructions) and record the balance
number.
2. Always have a piece of weighing
paper on the balance.
3. Before putting your object on the
balance, press the “Zero” or “Tare”
button to zero it with the weighing
paper.
4. Record every single digit shown in
the electronic display. This includes
all zeros!
weighing
paper
balance
number
“zero”
button
A top-loading balance
With this kind of balance, you do not need to estimate the uncertainty as what you did with
thermometer, ruler or graduated cylinder. The last digit shown on the balance display is the
“uncertain” or “estimated” digit. Therefore the balance shown above has an uncertainty/error
of 0.01 g. The measurements should always be reported as X.XX ± 0.01 g.
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Part IV. Mass Measurements
1. Determine and record the mass of the same nickel on
three different balances. Record the corresponding
balance numbers as well.
2. Determine the mass of 5.00 mL of water.
a. Since you have to use a container for water or
any other liquids, determine and record the mass
of an empty glass vial with its green cap on.
a. Without water
b. After 5.00 mL of water is transferred into the
vial, put the capped vial back onto the same
balance. Record this new mass measurement.
The difference between these two mass
measurements (without and with water) is the
b. With water
mass of water.
c. Empty the water in the vial into sink. Repeat
steps a and b for a second trial. You do not need
to dry the vial between trials.
But how do you measure 5.00 mL of water?
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Volumetric Pipette
pipette helper
non-pointed, normal end
Bulb:
Squeeze
out air.
Fill/Empty
Lever
pointed,
narrow end
volumetric pipette
1. Squeeze the bulb of the pipette helper to let air out.
2. Gently insert the non-pointed end of the volumetric pipette
into the pipette helper.
3. Submerge the pointed end of the pipette into water till it is
only a little bit above the bottom of the beaker. Keep it in
water when performing the next step.
4. Push the fill/empty lever up to bring the water into the
pipette. Stop filling when the bottom of the water meniscus
sits right on the fill line (see the picture on the left).
Fill Line
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Volumetric Pipette
5. Always point a filled pipette
downward so that the liquid does
not get into the pipette helper and
disable the helper. If the helper
seems to stop working, inform the
instructor immediately.
6. Move the pipette (still attached to
the helper) so that the pointed end is
now inside the vial as shown.
7. Push the fill/empty lever down to
discharge the water into the vial.
8. Do not worry about the last bit of
water remaining inside the pointed
end. It is not included in the 5.00mL volume.
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Lab Report
• Work on the post lab questions carefully and practice more to fully
understand how to determine increment and uncertainty.
• Follow the significant rules when doing calculation.
a. When subtracting or adding, pay attention to the numbers of
decimal places instead of the numbers of significant figures.
Example:
vial with water: 22.79 g (2 decimal places)
empty vial:
- 17.89 g (2 decimal places)
water:
4.90 g (2 decimal places)
b. When multiplying or dividing, count the numbers of significant
figures.
Mass
4.90 g ( 3 sig. fig. )

 0.980 g/mL ( 3 sig. fig.)
Example: Density 
Volume 5.00 mL ( 3 sig. fig. )
• Note: Conversion factors (such as 1.094 yd/m) have unlimited
numbers of significant figures, i.e. they do not determine the number
of significant figures in the final result.
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• Empty the water in the beaker and the green-capped vial into
sink and return them onto your bench.
• Put the used volumetric pipette into the used pipette holder
with the pointed end pointing upward.
• Wipe your bench with wet sponge.
• Ask your instructor to sign you out
• Next lab will be an application of what you
learned in this lab. You will be making
mass, volume, and temperature
measurements on an unknown liquid. Based
on its density, boiling point, and solubility,
you will be able to determine its identity.
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