Hey guys, Dr. Caddell, and this is the Freezing Point Depression lab.
The idea behind the experiment today is that whenever you have a pure substance;
it melts, freezes, same thing; at a certain temperature, and it boils at a
certain temperature. Whenever you dissolve something in that
substance, you have a solution. Now that solution consists of the solute, the
thing that's dissolved in it; and the solvent, the thing that it's being
dissolved in. So if you take some salt and put it into water, water would be the
solvent, salt would be the solute. And whenever you add a solute to a solvent,
the freezing point-- the melting point-- of that solvent goes down. It lowers, every
single time. And by the way, also, the boiling point raises. This is the reason
why they put salt on the roads up in the mountains where it snows, because you
put salt on the roads, basically you made a solution whenever it snows, and now
the freezing point of that solution is lower than the freezing point -- than the
temperature all around it. Everything around it is going to be about 32
degrees Fahrenheit, and it's not going to (freeze) there. It's gonna stay liquid
and not iced over. So what we're gonna do is, we're going to use the freezing
point depression equation; that's this guy right here. And what it says is this:
The delta Tf, that means the change in the freezing point, I'll explain that
just a second; is equal to -m kf. I'm gonna tell you what each of these
terms means. So the delta Tf is this. It's the change in the freezing point when
you add that solute to the solvent. So water freezes at zero Celsius when it's
just water. If you add some salt, it freezes at some lower temperature.
-1, -2, something like that. Depends on how much salt you put in it.
And now the delta Tf would be the freezing point with the solute in it--
with the, say, the salt in it-- minus the freezing point without the the solute,
just the pure solvent. So that's what delta Tf is. 'm' as a unit of concentration
called 'molality'. Not 'molarity', but 'molality'. And this, guys,
this is an important definition. This is the definition of molality: Moles
of the solute (the thing you're dissolving in there), divided by kilograms
of the solvent (thing you're dissolving it in). And so what we're going to do--
Oh, and the k sub f. This is just a constant, and it depends upon what the
solvent is. So I give you the value for k sub f in the lab. So you'll know what
that is. So we know this. What we're going to do
in the experiment is, we're going to measure our delta Tf. I'll tell you how
we do that in just a minute. But what that means is, then you can
solve this equation for the molality. That's gonna be some number, because we're
gonna measure the two temperatures, we're gonna know this, we're gonna find
what the molarity of that solution is. And that's going to be-- the molality is
going to be the moles of your unknown, divided by the kilograms of lauric acid,
which is what you're dissolving your unknown in. Once we know the molality,
it's real easy to find the moles of the solute (moles of your unknown), because
the solvent, the lauric acid, we're gonna weigh that. And so we'll know how many
grams, and that means how many kilograms of our solvent we have. So I
just take this number here, multiply it by the kilograms of lauric acid that we weigh
out. This cancels, and what we have left is the moles of solute. And so that's really
nice, because if we can find the moles of the solute, guys; our goal in this whole
experiment is to find the molar mass of our unknown, our solute. And the molar
mass, remember, this is grams over moles. Well this is how we get the moles.
We take, you know, the molality that we get from this equation, multiply it by the
kilograms of the solvent (the lauric acid), that gives us our moles; which
goes down here. The grams, that's easy. We're going to weigh the solute. And once
we know that, we have everything we need.
So remember that-- this is-- sometimes
people forget this, or get confused. The solute is the unknown, the solvent
is the lauric acid. And so let's go over and get started doing the experiment.
All right guys, so we're basically going to
follow the same process four times. What we're going to do is, we're going to
determine the freezing point of the pure solvent, which is lauric acid, two times.
And then we're going to add some of your unknown, which is the solute, and do the
same thing two more times. So for the the lauric acid, we're going to weigh out
some lauric acid first. Now what we're gonna do is, gonna use a large test tube.
We don't care how much to test tube weighs. We're gonna put in a beaker, we
don't care how much the beaker weighs.
So what we do is, we put the beaker with the empty test tube on the balance,
just like before. Close the door, tare the balance.
So now balance should say zero with the beaker and the test tube on it. Once you
do that, take it out. So it zeroed out, tared out, and we're going to
fill this guy up, pretty much all the way.
We're gonna see how much it weighs. However much it weighs when we
get it filled up, with just a little space at the top to put the rubber
stopper in, that's how much it's going to be. The reason we want to make sure that
it's as full as we can get it is that when this stuff melts, it actually
shrinks. It condenses, and the level goes down, and we need to make sure that we
have our thermometer all the way down inside of it,
because we're going to measure its temperature.
So now what I'm going to do is, I'm gonna take my rubber stopper
that I have. It's a one-hole rubber stopper, and we always do this with
rubber stoppers before we put them in glassware: There's a wide end and a
narrow end. We take the wide end and make sure it does not fit in the hole,
because if it does, when we turn it around this way and put it the way it's supposed
to go, it'll go all the way in and get stuck. You'll have to break the glass to get it out.
But I have that about as full as I can get it, with the stopper in there.
Now I'm not gonna weigh the stopper, because we just care about how
much this weighs. Put it back in, close the balance,
read that number from the balance. That's the mass of your solute (mass of lauric
acid), which is A1. That number is A1. I'm gonna take it out, put the rubber stopper
in, and I'm gonna put it in this boiling water, holding it with this ring stand.
Test tube clamp.
Now this is hot, so if I have to move it, I'm not going to touch it with my hands,
I use to beaker tongs. And that's what it's for. To move it around.
Submerse it as much as I can. It
doesn't matter, guys, that it's not all the way submersed. There's a
little bit of lauric acid on top. 'Cause you'll see, as it melts, it's gonna
shrink down. Okay so once that has finished melting, we're gonna take it out
and we're gonna put the thermometer in there, and we're gonna agitate it, and
we're just gonna record the temperature in this table every 10 seconds until it
solidifies, and then the temperature stays the same and starts decreasing again.
Once the temperature has gone down, stays the same and starts decreasing;
once we hit the decreasing part, we can stop. What happens is, whenever
anything is melting or freezing-- or, by the way, boiling or condensing-- the
temperature always stays the same during that entire phase change.
That's a good thing to know.
So you can see, it's almost melted right there. And we'll be able to
tell, it's real easy because we can see the white lauric acid. When it's all
melted, it'll be clear. It looks like water. It's a clear liquid.
Alright guys, so now that the lauric acid has all dissolved, we're going to start
recording our temperatures every 10 seconds as it freezes. I take it out. This
is really hot over here, so we're just going to lift it up over here. This is not
hot. Take my thermometer, using a digital thermometer; place it in here, and
just keep on shaking it and recording the temperature every 10 seconds.
Alright guys, so now that the lauric acid has completely frozen, temperature's
stayed the same for a while and then started dropping down, we can stop.
So we recorded our temperatures in our data table, but the only number that we
need is the temperature that it stayed at the longest. That's what we're gonna call
our freezing point. That's the number we need, that will be the freezing point of just
the solid. So what we're going to do now is, we need to melt this again.
Take that out. So I put it over here,
and as we're waiting for it to melt again, we are going to weigh out our unknown.
Our unknown is the solute, that's what it looks like. And so we're gonna
take a weigh boat right here, and put it on the balance. Tare it out, you know,
because we don't care what the weigh boat weighs.
It says zero with the weigh boat on it. Take it out.
Take all of our unknown, dump it into the weigh boat.
Take our unknown number off
of our test tube that are unknown came in, and just tape it in our data table. If
you have an unknown number, it will be A2 for you. Put this guy back on the
balance, close the doors. That number is the mass of the solute, which is the mass
of the unknown. That's going to be A3. Take it out.
Using a funnel, we're going to
add it to our test tube with our lauric acid in it. It doesn't matter
that it's not melted all yet.
Make sure it's all in there.
And now what we do is, we're going to wait for this to melt entirely,
and once it does, we're just going to follow the same process we did
with the pure lauric acid. Gonna take it out, agitate it, record the temperature
every 10 seconds until it solidifies, temperature stays the same for a while
and starts decreasing. The temperature that it stays at the longest will be the
freezing point of the solution. And that's going to be the experiment.
There we go.