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Unformatted text preview: beaker and contents and subtracting the mass of the beaker. This is known
as “mass by difference.” In this case, start by checking to see that the balance is level, and press
the “tare” button to set the balance to zero. Place your container on the balance pan, and record
the mass of the container. Once the container has the material in it, repeat the procedure (check
to see that the balance is level, press the “tare” button to set the balance to zero, place your
container on the balance pan, and record the mass of the container and material.)
Dakota State University Page 36 of 232 Basic Laboratory Procedures General Chemistry I and II Lab Manual Finally, just a couple of hints to improve your results when using a balance. First always
use the same balance. If the balance is off slightly, thee errors will usually cancel themselves out
if you are always using the same balance. Secondly, avoid drafts, vibrations or anything the that
might give you an erroneous reading. The best way to do this it avoid motion near the balance
when it is in use, avoid weighing objects when they are hot, and do not lean on the counter when
the balance is in use.
Graduated cylinders are used to measure volume. They are the most commonly used
devices for volume measurement in the lab because of their accuracy, speed and ease of use.
NEVER use the graduations on a beaker or flask for volume measurement; their accuracy is not
sufficient for laboratory use. Most graduated cylinders are accurate to three significant figures
(as opposed of flasks
and beakers that are
accurate to only two
significant figures, and
burettes and pipettes
that are accurate to four
begin, it is important to
note that graduated
cylinders are to be used
for measuring volume
only. NEVER use the
graduated cylinder to
mix reagents or to heat
With a liquid in
the graduated cylinder,
always read the bottom
(or top) of the meniscus. A meniscus is a curvature to the liquid caused by intermolecular forces
between the liquid and the glass. If you have attractive forces between the glass and the liquid,
such as water, the liquid will “creep up” the sides of the glass slightly to cause the normal
downward curvature. If these forces are repulsive, then the liquid will not move up along the
walls as far as the liquid, creating an inverted meniscus. Always look past the wall of the glass,
and read the volume at the center of the liquid.
Remember to estimate the last significant figure when reading the volume. This means
that you simply guess how far in between the two closest graduation lines the top of the
meniscus is. If it looks to you like the top of the meniscus is right on one of the graduation lines,
then record an extra “0” it the end of the recorded value so the reader knows that this is the case.
Dakota State University Page 37 of 232 Basic Laboratory Procedures General Chemistry I and II Lab Manual Pipette:
Like the graduated cylinder, the pipett...
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