Welcome to General Chemistry Lab.
Before we get started on how to use a balance, I want to point out the safety gear that I'm
wearing.
And we're going to wear this every time we're in lab.
The first one is my safety glasses.
And you'll notice that I have my regular eyeglasses on underneath.
Regular eyeglasses are not sufficient protection for you in the laboratory, so you have to
wear your safety glasses over your regular glasses every week.
If you don't have these on, you won't be permitted in the laboratory.
So that's one thing every week that I will be wearing.
The next thing you'll notice is that I have these protective gloves.
So you can select from Nitrile or latex.
Those are provided for you in the laboratory.
You want to make sure that they're not torn, or they're not going to serve you very well.
And you'll make sure that they fit, because, again, if they're too big, then you're not
going to be able to handle the instruments and the materials very well.
So the first thing I want to demonstrate is using our balance.
One of the first things you'll notice are all of these doors around.
Before I use the balance, what I want to do is clean it off.
I'm going to open the doors.
There's a paint brush that's lying either on the top of the bench or around the lab
bench near the balance.
And I want to brush it off, especially the tracks.
You can remove the pan carefully and brush around.
And then gently replace the pan.
We need to be really gentle with this instrument and treat it with care.
It is delicate.
You can read on the digital readout that it has a max of 420 grams, which seems like a
lot, but it's really not.
Before I start using the instrument, after I've cleaned it off, I'm going to close all
of the doors.
And I'm going to tare it, which means set the mass reading of the balance to zero.
On this particular model, it's a zero slash capital T button.
I'll go ahead and hit that.
For our first experiment, we're weighing pennies.
So I'm going to count out five.
And I'm going to gently set the penny roughly in the center of the pan.
You don't want to push down when you're putting something on the pan or when you're taking
it off.
These instruments are really delicate.
Now that I've closed my door and all of my doors are closed, I'm going to wait until
the digital reading has stabilized.
Once it's stopped changing, this is the mass that I'll record on my data sheet.
Again, I'm going to gently remove my penny, and I'm going to put it back.
I don't want to reweigh the same penny.
And you can close the door and check again that you're zeroed out or tared.
You can hit it again just for good measure.
The reason that I keep the door closed when I'm recording the mass is because sometimes,
there'll be a breeze, and the air pressure will be a little bit different, and it will
affect the reading.
Probably not by much, but we're measuring pretty small differences, so we want to be
pretty careful.
Again, my digital readout has stabilized.
It has stopped changing.
So this is the mass that I'll record for my second penny.
And carefully remove it.
I don't want to push down if I can avoid that.
Tare out just for good measure, even though the reading was already zero.
And I'll continue to weigh my samples until I have all the data for that experiment.
In the first experiment when we're weighing pennies, it's OK just to put the penny directly
on the pan.
But sometimes in later experiments, we'll be using powders or different granular material,
and we don't want to put that directly on the pan.
So we'll use weighing paper.
And I don't want to just put it in here.
Sometimes, it can bow out like this.
And we're going to be using relatively small masses, and so your powder or your salt will
just roll right off and get on the pan.
And we don't want that.
So a trick is to just fold it in half and then set it on the pan, again, roughly centered,
and close the door.
And I'm going to tare this out so that when I add my powder, my material, I'm only recording
the mass of the material and not the mass of the weight paper.
It seems like a small difference, but it will make a difference in our calculations.
We always want to do this before because if you do it after, it's impossible to scrape
off every single bit, every single crumble, every single granular fragment.
Once I'm done using the instrument, I want to clean it off again.
So I'm going to open the doors, and I'm going to use the paint brush really carefully.
I'm going to remove the pan.
A lot of times, the powder falls off and rolls under the pan.
So I want to sweep that off and return the pan.
And then, I want to make sure I sweep out the tracks.
So even though I'm just using pennies right now, these tracks can get dusty and sticky.
And what happens over time is that you have an instrument like this balance that is nearly
impossible to manipulate sliding these doors because over time, the material has just collected
in the tracks and looks like it's oxidized.
So that's our introduction on how to use the balance.