Hi everyone, welcome to the first video on the Gravimetric Analysis lab.
Let me first discuss the goal of the lab.
First, you will be learning a few experimental techniques in this lab: carrying out a precipitation
reaction, filtration, and heating with crucibles.
A second goal is to understand how the concept of percent composition can be used to determine
the identity of an unknown.
The term Gravimetric Analysis literally means analysis using weights.
So in this case, what we’re trying to do is to determine the identity of an unknown
by measuring masses of substances.
Gravimetric Analysis can be done on any type of chemical reaction.
In today’s lab, we will specifically carry this analysis out on a precipitation reaction,
which is a reaction that produces a solid product.
This picture shows an example of a precipitation reaction.
When you mix one reactant with another, you immediately see what looks like a yellow cloud
appearing.
The cloudy substance blocks your view, so that is a sign that it is a solid.
If you wait for some time, the solid will eventually settle to the bottom of the beaker
forming a sediment or more commonly called a precipitate.
Often, the sedimented solid can be collected using a technique called decantation, which
is used to separate the clear solution away from the precipitate.
The precipitation reaction you will carry out today is a reaction between an unknown
sulfate salt, which we give the formula MxSO4, and barium chloride, BaCl2.
One of the products in this reaction is barium sulfate, which is a precipitate, and a chloride
salt which is aqueous or soluble.
Our goal is to collect and measure the mass of the barium sulfate product.
Using that and the mass of unknown sulfate salt, we will be able to determine the identity
of the metal M in the unknown salt.
Note that for this idea to work, we must carry out the reaction with the unknown sulfate
as the limiting reactant.
This condition ensures that all the sulfate in the unknown will be converted to the barium
sulfate.
To determine the identity of the metal cation M, we will use the two definitions of percent
composition.
One way to calculate percent composition of an element in a compound is by taking the
molar mass of the element and multiply it by the number of moles of the element in the
compound and then dividing the result by the molar mass of the entire compound.
I call this the theoretical definition, because it can be calculated without doing any measurement.
You just need the chemical formula of the compound and the molar masses of the elements
from the Periodic Table.
There is a second way to calculate % composition, which I call the experimental definition.
Here, you use the actual mass of an element that you measure divided by the mass of the
compound.
Note the difference between mass, a measured quantity and molar mass in the previous definition,
which does not need to be measured.
Now I will show you how these two definitions can help us determine the identity of the
metal cation M in our unknown sulfate salt.
In this example, we measure 0.323 g of the unknown sulfate and react it to form 0.433
g of a precipitate.
This precipitate is our barium sulfate as the equation above shows.
You may wonder about the other pieces of information given here, the 50 mL of water and the 6 M
HCl.
These are experimental conditions that do not affect our calculations.
To determine the identity of the cation, we just need the two masses.
First we must determine the mass of sulfate in our unknown sulfate.
Because we assume that all the sulfate is converted to barium sulfate, the mass of sulfate
in the unknown is also equal to the mass of sulfate in barium sulfate.
The mass of sulfate in barium sulfate we can calculate by multiplying the percent composition
of sulfate in barium sulfate and the mass of barium sulfate.
The percent composition of sulfate in barium sulfate is just the molar mass of sulfate,
96.06 g/mol, divided by the molar mass of barium sulfate, 233.38 g/mol times 0.433 g,
which is the mass of the barium sulfate.
This gives us the following answer, with the third digit bolded to keep track of significant
figure.
Now we can calculate the percent composition of sulfate in our unknown compound, by using
the experimental definition of percent composition.
So we will take the mass of sulfate and divide by the mass of the unknown compound, 0.323
g to give us 55.17779%.
This number is useful because it should be the same percentage we get if we use the theoretical
definition, which is molar mass of sulfate divided by molar mass of unknown compound.
As you can see, if we know the percentage and the molar mass of sulfate, we can solve
for the molar mass of compound, which is 174.091778 g/mol.
From there, we can subtract out the sulfate to get the molar mass of the cations as 78.031778
g/mol.
To determine which cation we have, we need to use both the molar mass and charge information.
There are three possible cation charges that we can have, +1, +2 and +3.
The formula of the sulfate compound with each type of cation is shown here.
If we have a +1 cation, then we have two moles of the cation, so the correct molar mass must
be half of what we calculate, in this case 39.0 g/mol.
If we have a +2 cation, the molar mass is equal to the number we calculate, 78.0 g/mol.
If we try to use a +3 cation, it won’t work in this case because the total molar mass
we have is too small.
So the only options are either +1 or +2 cations.
We go to the Periodic Table and find that potassium has a molar mass of 39.1 g/mol and
it is a group I cation, so therefore it must be out answer.
To confirm, if we try to find an element with molar mass of 78.0 g/mol, the closest is Selenium
which is an anion not a cation.
So that’s how you use the concept of percent composition and mass data to determine the
identity of your unknown.
In the next video I will go through the experimental steps that you must perform for a successful
reaction.