CH-180
Chemistry in Our World
Introduction to Laboratory & Making Measurements
How do we know what we know about chemistry? Well, we might answer that
question by saying, "The instructor told us," or "It's in the book." But, true as those
answers may be, they tend to leave a thinking person unsatisfied. After all, there isn't
much difference between the questions, "How do we know what we know about
chemistry," and "How do the instructors and authors know what they know about
chemistry?" To be sure, the best answer to our question is this: "We know what we
know about chemistry by observing the behavior of substances." We then use our
observations to draw meaningful conclusions that we know as chemistry. Chemists can and do - make observations in many different places, but chemists most often make their
observations in laboratories, rooms that are specially equipped to make observation easier
and safer.
A. Safety Equipment and Procedures
A major reason why chemists prefer to observe the behavior of chemicals in a laboratory
is that the laboratory is fitted with safety equipment designed to minimize dangers
presented by the chemicals or procedures. The instructor will show you the location and
proper operation of the safety equipment in our lab. In general, however, you are wise to
remember the following guidelines:
1. Most lab accidents are the result of ignorance or indifference. Develop the habit of
reading the handout for the experiment before coming to lab, and generate an interest in
doing the experiment.
2. Follow safety instructions when in lab. If this means reading an instruction six times
and then consulting the instructor for an interpretation, do so. If a neighbor is obviously
performing a procedure contrary to instructions, don't hesitate to warn the neighbor or
call the instructor for help!
3. WEAR YOUR GOGGLES throughout the experiment, (unless the instructor
specifically tells you otherwise). Failure to do so could result in a zero for the
experiment.
4. Do not eat, drink, or smoke in lab. Failure to comply could result in a zero for the
experiment.
5. Tie up long hair before coming to lab.
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6. Do not wear jewelry, such as rings, bracelets, watches, etc. in lab - not only could the
jewelry become lost or damaged, but it could also trap chemicals against the skin, causing
a burn or rash.
7. If possible, bring into the lab only those books and papers which you will use while in
lab.
8. Don’t wear open-toed shoes to lab: broken glass and spills can be very hard on bare
feet!
9. If a student is pregnant she should notify the instructor or TA. Some chemicals have
dangerous effects on the fetus during pregnancy.
10. Report any accidents to the instructor or TA immediately. Chemicals spilled on your
skin or in your eyes should be flushed with a large amount of water. The instructor will
arrange for transportation and medical attention.
11. The instructor and TA will provide instructions at the beginning of each lab period
concerning waste disposal. Don’t dump anything down the drain or put anything in the
trash unless specifically told to do so.
12. Only work which is assigned by the instructor and TA may be done in the lab. You
MAY NOT work without supervision.
13. Discard excess reagents. Never return them into the reagent bottles. Don’t put pipes
into reagent bottles.
Lab can be a safe, enjoyable, and rewarding experience if you PREPARE for the
experiment, PAY ATTENTION to what is going on, and USE COMMON SENSE.
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B. Making Measurements in Lab
The equipment that you have in the lab drawers will help you to observe the behavior of
substances. Often, we will make our observations in the form of measurements, that is,
we will use an instrument to tell us something about a substance. Three instruments
especially important to us in this course are the
Analytical Balance
Graduated Cylinder
Thermometer
Today, we'll practice using all three. Perhaps the most important thing here is that a
measurement always involves an estimate. For us, this means that the last digit in each
measurement is estimated. There is some uncertainty associated with every measurement
that we make.
The Analytical Balance helps us make meaningful observations about the behavior of
substances by making precise mass measurements.
Our analytical balances measure mass in grams (a metric system unit) to the nearest
thousandth of a gram (0.001 g). Thus, any analytical balance measurement that we write
should indicate units of grams and should have three digits to the right of the decimal
point, to show that our balances are good enough to give the mass to thousandths of a
gram, rather than hundredths, tenths, etc. E.g.,
6.045 g
109.400 g
0.040 g
are all masses that can be measured on our balances. The instructor will show you how to
use the analytical balance, if necessary. The electronics in the balance perform an
estimation procedure to give us the rightmost digit in the digital display.
The Graduated Cylinder is calibrated in units of milliliters
(mL), another metric unit. Unlike the balance, the graduated
cylinder has no electronics; this means that we must perform the
estimation procedure to give the last digit in the measurement.
Note that, in most cases, the graduate cylinder is numbered in
whole milliliters; each mL is usually divided into five equal parts (0.2 mL). We measure
the volume of water-based liquids by reading the graduated cylinder's markings at
the bottom of the meniscus. (The meniscus is simply the convex or concave surface that
the liquid makes in the cylinder).
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The Thermometer measures temperature in degrees Celsius (°C). Unlike medical
thermometers, your thermometer should not be shaken down. Read it as you read the
graduated cylinder, but estimate the temperature to the nearest 0.1°. Do you see why we
read the temperature to the 0.1's place? (The smallest marked division is in the 1's place,
meaning that our estimate must be in the 0.1's place: a rule for instruments is that the
estimate will always be one place to the right of the place in which the smallest
marked division is found.)
D. More on Uncertainty
Scientists know that every measurement that we make involves some degree of
uncertainty, simply because the measuring devices themselves are limited in precision by
their design and the amount of care taken to manufacture them. As you will learn in
class, scientists also understand that knowledge of how much uncertainty is in a
measurement will influence the way that the measurement's meaning is interpreted.
Scientists have therefore developed the system of significant figures to conveniently
communicate the uncertainty of measurements.
One of our main goals for CH-180 is to explore what it means to think and act like a
scientist. To that end, we will use significant figures for all of our measurements.
Getting correct significant figures is easy if the instrument has a digital readout: simply
write down every digit, even if it is a zero. For example, if your analytical balance
displays 2.500 g, you should write 2.500 g, NOT 2.5 g or 2.50 g. The two zeros in 2.500
g communicate to anyone who reads the number, that the balance was of good enough
quality to read masses all the way to the thousandths of a gram, and not just tenths or
hundredths. If the instrument has a scale instead of a digital readout, simply read the
number according to all the marked divisions, then estimate the last digit. For example,
The thermometer's mercury is between 25°C and 26°C; estimating that it is about 4/10 of
the way between 25°C and 26°C gives us a final reading of 25.4°C. Writing in the "4"
tells anyone who reads the number, that the thermometer was good enough to read to the
tenths of a degree Celsius.
Now answer the questions and complete the measurements presented on the next page.
Procedures are not provided for the measurements; just as professional scientists must do,
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you will have to develop your own procedures from your knowledge of the instrument
used and the property being measured.
A Practice Problem, With Answers
A group of four students were performing a lab experiment for which they were given the
following directions:
Weigh 2 g sodium chloride to the nearest thousandth of a gram.
Each of the students interpreted the directions differently:
a. Ajax placed a weighing boat on the balance and read a mass of 1.000 g. He then placed
enough sodium chloride into the boat to bring the total mass to 2.000 g.
b. Brunhilde placed a weighing boat on the balance, and then added sodium chloride until
the balance read 3.000 g.
c. Celestine placed a weighing boat on the balance and read a mass of 1.01 g. She then
added enough sodium chloride to bring the total mass to 3.00 g.
d. Dionysius placed a weighing boat on the balance and read a mass of 1.209 g. He then
added enough sodium chloride to bring the total mass to 3.304 g.
Circle the letter corresponding to the student who best interpreted the directions that were
given. Then, analyze each student's interpretation, clearly stating why you concluded that
the student did or did not follow the directions. Be as specific as possible in your
analysis.
Analysis:
a. Poor choice. Although the measurements were made to the 0.001's place, Ajax
measured out only one gram of sodium chloride, not two.
b. Poor choice, because we don't have enough information: we don't know the
weight of the weighing boat that Brunhilde used, so she may or may not have
weighed out 2 g sodium chloride. Moreover, we don't know if the weighing boat was
weighed to the 0.001's place.
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c. Poor choice. Although Celestine did weigh out 2 g (1.99 g rounds to 2 g), she did
not weigh to the 0.001's place, as required.
d. Best choice. All measurements were made to the 0.001's place and the mass of
sodium chloride (2.095 g) rounds to 2 g, when taken to one sig fig.
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Making Measurements Report
Name _____________________
Date __________________________
Questions:
1. Sketch a meniscus. Clearly indicate the point on the meniscus where a volume
measurement should be made by including an arrow in your sketch.
2. Write the temperature (including the estimate) given by the thermometer
shown here:
Temperature is:
C
3. Using one of the analytical balances in our lab, Cal Chemical determined that a 1978
penny weighed 2.944 g. Cal’s girlfriend, Anne Analyst, weighed the same penny on a
balance that could be read to the tenths of a gram. What mass did Anne obtain on her
balance?
g
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Make the following measurements, and write the number - including units - in the
space provided on the right. Some of the measurements can be made at your lab
station; others must be made elsewhere around the room. REMEMBER: all
measurements should be made to the limit of the instrument, i.e., record all digits
read from the scale.
1. Room Temperature ____________
2. Temperature change due to evaporation ____________
3. Volume held by a 50 mL beaker ____________
4. Volume of red liquid in a graduated cylinder ____________
5. Volume of green liquid in a graduated cylinder ____________
6. Volume of 50 drops of water ____________
7. Volume of 3 pennies ____________
8. Volume of a penny ____________
9. Temperature change due to adding 10 mL ice water to 10 mL hot water
____________
10. Temperature change due to adding 10 mL ice water to 20 mL hot water
____________
11. Mass of a 1989 penny ____________
12. Mass of a 1980 penny ____________
13. Mass of a 1990 penny ____________
14. Mass of 3 pennies (1980, 1989, 1990) ____________
15. Mass of 10.00 mL water ____________
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Describe in detail the procedure that you used to obtain your measurement in #15. The
detail must be sufficient to make the measurement to the correct number of sig figs.
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