|Learning Objective: Modern analytical
balances are electronic devices designed to measure mass precisely
and accurately to the ten-thousandth of a gram (typical). Here you
will learn how to use an analytical balance to determine the mass
of an object and a substance.
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|We will use an analytical balance
with an uncertainty of 0.0002g to measure small amounts of solids whose
mass must be accurately known (to the ten thousandth
g). The balances are always turned on and calibrated, and no adjustment
is necessary. They are very sensitive, so treat
them with care. They are designed to give an accurate mass measurement within
seconds after placing the object on the pan. To use an analytical balance,
complete the following steps.
|The display should read 0.0000g.
If it does not, press the Rezero button or 'T' (Tare) bar beneath the display
once, and wait for zero to appear. Be
patient; every touch represents a command which the balance will
follow. If pressed too many times, it will go into 'Busy'
mode until it has answered all the commands.
|Since reagents cannot be weighed
directly on the pan, you will first need to weigh a container. Record the
first steady reading with a 'g' on the right side. Readings for light containers
may fluctuate, so take the first reading with a 'g' on the right side that
stays on the display for 3 seconds or longer.
Next, add the substance whose mass you want to measure to the container. Again, record the first steady reading that stays on the display for 3 seconds or longer. The mass of the substance is the difference between the second reading and the first reading. Once you're finished, gently remove the container with the substance and let the display return to zero. If it does not, press 'Tare' once. Clean immediately any spilled solid using the brush stored inside the balance. Close all open doors.
When recording readings, keep in mind the following to ensure the most accurate measurement:
1) Keep the balance doors shut to prevent any draft in the lab from effecting the balance reading.
2) All objects to be weighed should be at room temperature. Objects that are either too warm or too cold will have an effect similar to a draft and result in an inaccurate reading.
3) Avoid leaning on the bench where the balance resides, otherwise you may tilt the balance and get an inaccurate mass reading.
4) Ensure all containers and reagents are dry when placed inside the
balance. Any moisture present will evaporate over time, causing the mass
to decrease constantly. If you notice a declining mass measurement, remove
the container from the balance and dry it with a Kim wipe, and then place
it inside the balance again.
|Since a slight difference
in the balance point of each balance is normal, it is strongly recommended
that you use the same balance for all measurements
throughout an experiment. You cannot use beakers or flasks greater
than 125 mL on the pan. The balance has a capacity of 110.0 g. If
an "H" appears on the display, it indicates that what
you are trying to weigh exceeds balance capacity.
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