Video Transcript
Put the following statements in the
correct order to describe how a balance should be used to measure the mass of a
solid. (a) Set the balance to zero. (b) Record the final mass of the
sample. (c) Place the container onto the
mass balance. (d) Weigh out the substance into
the container.
A balance is an instrument used to
measure mass. There are many types of electronic
balances that we may encounter in the chemistry laboratory. The most commonly used electronic
balance is accurate to two decimal places and is known as a two-figure balance or
top loading balance. On top of the balance is a metal
pan, where we place the objects we wish to mass. A digital display shows the mass to
two decimal places. The zero or Tare button allows us
to set the display to zero. The question asks us to put the
following statements in the correct order to describe how a balance like this one
should be used to measure the mass of a solid.
Let’s discuss how we would use this
two-figure balance to measure the mass of a solid like copper sulfate. Firstly, we don’t want to place the
chemicals directly on the metal pan, so we’ll need a container like a beaker or a
disposable plastic weigh boat. If the only step we take is to
place the weigh boat and copper sulfate directly on the balance, then the mass
displayed is the mass of the copper sulfate and the weigh boat. But we just want the mass of copper
sulfate, the solid. So we should start by placing an
empty weigh boat on the balance. Then, we should press the zero or
Tare button. This sets the balance to zero and
effectively tells the balance to ignore the mass of the weigh boat.
Next, we should add our solid to
the weigh boat. It’s best practice to remove the
weigh boat from the balance before adding the solid. This is to prevent spilling
material onto the balance pan and potentially damaging the mechanism inside. We’ll notice that when we remove
the weigh boat from the balance, the balance will display a negative mass. The mass is negative because the
balance will subtract this mass from the mass of whatever is placed on the pan. So if we now place the weigh boat
and the copper sulfate on the balance, the balance will automatically subtract the
mass of the weigh boat and display only the mass of the copper sulfate.
We can now record the final mass of
the sample. We now know that the steps in order
are (c) place the container onto the mass balance, (a) set the balance to zero, (d)
weigh out the substance into the container, and (b) record the final mass of the
sample. So the correct order of the
statements to describe how a balance should be used to measure the mass of a solid
is (c), (a), (d), (b).