Hi everyone, welcome back to this second video in the Density series.
In the first video we discussed how to use graduated cylinder to measure volume and then
later on calculate density.
You’re now going to repeat the experiment for the other two columns.
For the second column, you will do the experiment again but this time you will use a 10-mL volumetric
pipet to get your sample of water.
You will pour out the water sample you have in the small beaker.
Remember that was for the graduated cylinder measurement and we’re done with that.
Dry the small beaker well with napkin and then measure its mass again and record it.
You may wonder why we don’t use the same number that we had for the graduated cylinder.
The reason is because fluctuation in that mass, so it is experimentally more correct
to re-measure the mass of the empty small beaker.
Once you have the mass of the empty small beaker, you want to transfer water into it
but now you will use a volumetric pipet
The volumetric pipet is a glassware where there is only one mark.
This is different compared to the graduated cylinder which has a lot of marks.
Because the pipet only has one mark, the goal to get the liquid’s meniscus exactly at
the mark.
When you get the liquid’s meniscus is at the mark, you're going to write down that
your volume of water is 10.00 mL.
To get the water into the pipet, you will use a pipet pump, which is shown here in various
colors.
First, you unscrew the collar of the pipet pump, turning it counterclockwise.
Then insert the pipet into the pump and turn the collar clockwise to close it.
Now the pipet is securely in the pump and there is a tight seal ensuring correct volume
delivery.
Submerge your pipet tip below the water level in your medium sized beaker.
Start turning the thumbwheel of the pipet pump up and you will see liquid slowly rising
in the pipet.
Keep doing this until the liquid’s meniscus is about an inch above the pipet mark.
Then move the tip of the pipet out of the water and turn the thumbwheel down until the
liquid’s meniscus is exactly at the mark of the pipet.
At this point you have 10 mL in your pipet.
Move the tip of the pipet to the inside walls of the small beaker.
Press the fast release lever, located on the side of the pump.
This will allow the liquid to empty out into the beaker.
In the lab, you may see two different types of volumetric pipets.
If the letters TD are written on the side of the pipet, this means that the pipet will
have a small amount of liquid left in it when 10 mL of liquid has been delivered.
So if you’re using a TD pipet, do not push out that last bit of water.
If the letters TC are written on the side of the pipet, this means all the liquid has
to be pushed out of the pipet in order for it to deliver 10 mL.
So look at the pipet to see which one you have.
Again, don’t forget that there is only one correct volume you can write for the “actual
volume of water used” when you’re measuring with a volumetric pipet.
That volume is 10.00 mL.
There's no other number you can put here and that's the only way to use the pipet.
The last glassware you will use is a 50.00 mL buret.
A buret has many marks just like a graduated cylinder, so you will read the volume with
the steps I described earlier for the graduated cylinder.
First you’re going to figure out the size of each mark, then find out where the meniscus
is, and to finish add one more digit as an estimate of where you think the meniscus actually
is.
A few things are important to keep in mind when using the buret.
First, you fill the buret with your liquid by pouring the liquid through a funnel inserted
into the buret.
Make sure the stopcock is closed otherwise you’ll have a major leak, because the liquid
will just stream out of the buret.
Typically, you will see an air bubble trapped at the tip of the buret, which is shown here.
If this is the case, what you read from the buret will be wrong, so you must flush out
the air bubble first.
To do this, simply open the stopcock and let the flow of liquid push out the air bubble.
Once the bubble clears the buret, you can read the volume correctly.
Remember that a buret starts with zero at the top and its maximum value at the bottom
so it's flipped compared to the graduated cylinder.
To read it, simply find the meniscus and read its volume.
For example, in the example I’m showing here, the initial reading of the buret is
9.65 mL.
What you care about how much liquid is dispensed or taken out from the buret, not how much
liquid is in the buret.
The 9.65 mL we read earlier merely tells us the initial position of the liquid.
In your experiment, you want to dispense about 12 mL of liquid water from the buret into
the empty small beaker, so determine roughly where you should stop and open the stopcock
and let the water flow out.
So if you start at 9.65 mL, you may want to end around 21 or 22 mL.
In the example if you read the volume, it is 24.15 mL.
So if I subtract the initial from the final what I get is 14.50 mL.
Like in the other parts of this lab, the significant figures in your calculation must be correct.
In your lab report, you will write the 14.50 ml here.
Again your number should be closer to 12 so you would write it here for the actual volume
of water.
Now in your lab report there’s no designated space to write the initial and final volumes
of liquid in the buret, but I want you to write those values next to the third column
so I know how you get the actual volume of water used.
Now what you will do is empty the water out into the small beaker and measure the mass
again to calculate the mass of water.
Divide mass by volume to get the density.
Once you complete the measurement and calculation for all three data columns, show your results
to your instructor to check that your calculation and significant figures are correct.
After your results have been confirmed, you will write the density values on the board
in a table for the density values of the entire class.
Once all the groups fill in their values, each student will copy the results to the
class table in their lab report.
The purpose of this table is for you to calculate the mean and standard deviation for each glassware.
Recall that you learn how to do this calculations in your experiment with Excel the last time.
You want to compare the precision of the glassware by calculating the percent error in precision.
The larger the percent error, the less precise the instrument.
The way you calculate the percent error is by taking the standard deviation and dividing
it by the mean for each instrument.
Remember that for both the cylinder and buret, you want to use about 12 mL of water as your
sample.
For the volumetric pipet, the volume is 10.00 mL when the water’s meniscus is brought
to the mark on the pipet.
Pay attention to significant figures, it is a major part of your grade in this experiment.
Ok, in the next video, I will describe the other two parts of the Density lab.