Video Transcript
In this video, we will learn how to
use volatilization gravimetry to calculate the quantity of analyte in a sample or to
determine the formula of a hydrated compound.
What is volatilization
gravimetry? Volatilization, which comes from
the word volatile, means the conversion of a substance or substances to the gas
phase. Typically, a sample, whether it be
a compound or a mixture, is either heated or allowed to undergo a chemical
reaction. And the volatile components are
liberated and are thus separated from the remaining components of the sample. Gravimetry or gravimetric analysis
is an analytical method used to measure mass or change in mass to quantify an
analyte. Putting these two definitions
together, we get a new definition. Volatilization gravimetry is a mass
analysis method that uses thermal or chemical energy to separate substances in order
to measure their masses.
If the mass of the original
starting sample before separation is known and the volatile components are driven
off through heat energy or a chemical reaction, then the mass of the substances
remaining after separation can be determined by weighing the vessel. If we then find the difference
between these two masses, we get the mass of volatile substances liberated. This simple calculation is the main
principle of volatilization gravimetry. Examples of volatile compounds
which may be released during chemical or thermal decomposition of a sample include
nitrogen gas, chlorine gas, carbon dioxide gas, and water vapor. Now, let’s have a look at the loss
of water vapor from a hydrated salt as a specific example of volatilization
gravimetry.
When a salt contains water
molecules within its structure, we say the salt is a hydrated salt. Water molecules that form part of a
salt crystal lattice structure are specifically referred to as water of
crystallization or sometimes water of hydration. For example, the hydrated salt
copper(II) sulfate contains water molecules within its structure, giving the
crystals a blue appearance. The formula for this compound is
CuSO4.5H2O. Commonly hydrated copper sulfate
has five water molecules within its crystal lattice per one formula unit of copper
sulfate. The correct name of this compound
is therefore copper(II) sulfate pentahydrate although we often just refer to this
compound as hydrated copper(II) sulfate. We can determine the mass of water
of crystallization in a salt, such as this, using volatilization gravimetry.
If we know the mass of the hydrated
salt and we remove the water of crystallization using thermal energy by heating, the
water will escape as a vapor leaving behind a white powder. This is dehydrated copper(II)
sulfate or anhydrous copper(II) sulphate, where anhydrous refers to a salt that does
not contain water of crystallization. So, anhydrous copper(II) sulfate
has the formula CuSO4. The mass of this dehydrated salt
can be determined by weighing. The mass of the anhydrous salt is
smaller than the hydrated salt because of the loss of water. The mass of this water lost can be
determined by the difference in the masses between the hydrated and dehydrated
salts. The balanced chemical equation for
this reaction is CuSO4.5H2O solid being heated to give CuSO4 solid plus 5H2O
gas.
Incidentally, this is a reversible
reaction in reality. The reverse reaction is promoted by
the addition of water and cooling. The setup of this volatilization
process is as follows. An iron ring is clamped to a retort
stand. A clay triangle is placed on the
iron ring, and a crucible containing the hydrated salt is placed onto the clay
triangle. The contents are strongly
heated. The crucible lid is left slightly
open to allow water vapor to escape. Periodically, the lid is put back
in place, and the heat is removed and the crucible is allowed to cool. The crucible and its contents are
weighed and the mass recorded. Successive heatings and coolings
are repeated over and over until the mass of the crucible and its contents is
constant.
At this point, we know that all the
water has been removed, and the contents of the crucible appear white instead of
blue. This unchanging constant mass of
dehydrated copper sulfate is what we will use in calculations. Volatilization gravimetry is also
useful to determine the coefficient for the water of crystallization if it is
unknown. Since we know the mass of the
starting hydrated substance, and because we can determine the mass of the dehydrated
salt after heating to constant mass, we know that we can determine the mass of water
vapor which was liberated during heating and that this is the same mass value as the
mass of water of crystallization in the original hydrated salt. We can then determine the molar
mass of water. And using the mass and molar mass
together, we can determine the number of moles of water.
We can use the key equation number
of moles is equal to mass divided by molar mass. Then, we can use the number of
moles of water to determine the number of moles of the hydrated salt. We can do that using the molar
ratio one as to n. We are going to practice this type
of calculation in a moment. But before we do that, let’s look
at one more useful piece of information we can get using volatilization
gravimetry. We can also use the various masses
determined from a volatilization experiment to calculate the percentage of water of
crystallization in a hydrated salt.
The percentage water of
crystallization in a hydrated salt is equal to the mass of water of crystallization
divided by the mass of the hydrated compound times 100 percent. And we’ve already seen that these
masses can be easily determined using volatilization gravimetry. Now, it’s time to practice.
A sample of cobalt(II) chloride
hydrate CoCl2⋅𝑥H2O is heated until its mass remains constant. For every 1.00 grams of cobalt(II)
chloride produced, 0.831 grams of water is liberated. What is the value of 𝑥, where 𝑥 is
an integer? And we are given the molar mass of
cobalt, 59 grams per mole; for chlorine, 35.5 grams per mole; for hydrogen, one gram
per mole; and oxygen, 16 grams per mole.
We are told that a hydrated salt,
cobalt(II) chloride, is heated until constant mass. The name cobalt(II) chloride
hydrate and the formula do not tell us how many molecules of water they are per unit
of cobalt chloride. We are only told the number of
moles of water of crystallization in terms of x. Heating a hydrated salt to constant
mass typically involves heating a salt strongly in a crucible with the lid off to
allow the water of crystallization to escape.
The reaction equation for this
process is CoCl2⋅𝑥H2O solid being heated to give CoCl2 solid plus 𝑥H2O gas. Over time, all the water of
crystallization is removed. The hydrated salt is converted to
an anhydrous salt. An anhydrous salt contains no water
of crystallization. During cooling and weighing, the
lid of the crucible must be in place, and this is to prevent water vapor from the
air reentering the crucible.
This is important because this
reaction is reversible, and we want to prevent the reverse reaction from
occurring. Otherwise, the masses of each
substance are unknown. We need to ensure that cobalt(II)
chloride is indeed fully anhydrous. To solve this problem, we can put
the data that we are given and that which we need to find into a table. This table is not absolutely
necessary, but sometimes it helps when we are dealing with a lot of information.
We are told that for every 1.00
grams of cobalt(II) chloride produced, so let’s put this mass into the table under
CoCl2, 0.831 grams of water is liberated. And so, this is the mass of
water. The words “for every” tell us that
1.00 grams of cobalt(II) chloride and 0.831 grams of water are not necessarily the
actual masses of these substances produced, but rather the ratio in which they are
produced.
We know that the stoichiometric
coefficients one as to 𝑥 is the mole ratio of these substances. So first, we need to convert the
masses of these products to moles. We start by calculating the molar
mass of cobalt(II) chloride, which is equal to the molar mass of cobalt plus two
times the molar mass of chlorine since there are two chlorines. Substituting in the molar mass of
cobalt, which was given to us 59 grams per mole, and that of chlorine, 35.5 grams
per mole, we get a molar mass of cobalt(II) chloride of 130 grams per mole. We can put this value into the
table.
The molar mass of water is equal to
two times the molar mass of hydrogen because there are two hydrogens plus the molar
mass of oxygen, and these values are given to us. So, substituting one gram per mole
for hydrogen and 16 grams per mole for oxygen, we get 18 grams per mole, the molar
mass of water, which we can put into the table.
The next step is to calculate the
number of moles of cobalt(II) chloride and water. We can use the key equation number
of moles is mass divided by molar mass. So, for cobalt(II) chloride, taking
its mass and molar mass from the table, we get 1.00 grams divided by 130 grams per
mole, which is equal to 0.00769 moles. For water, the number of moles is
equal to its mass, 0.831 grams, divided by its molar mass, 18 grams per mole, which
gives 0.04616 moles.
Since the ratio of the moles of
cobalt(II) chloride as to water are the same as the stoichiometric ratio one as to
𝑥, we can now solve for 𝑥. How we do this is we take the moles
of cobalt(II) chloride as to the moles of water and divide each value by the moles
of cobalt(II) chloride. These two values cancel, giving us
one. The moles units cancel on both
sides, and solving this division here, we get 6.00166.
So far, in this calculation, we
have not rounded off, but now it is time to do so. We need to round off to a value
with no decimal places since 𝑥 cannot be a fraction. We cannot have a fraction of a
water molecule. And we are specifically told that 𝑥
is an integer. Finally, we get an answer of one as
to six. And this is the mole ratio of
cobalt chloride as to water. These values are also the
coefficients in the hydrated compound formula. Therefore, the value of 𝑥 is
six. And the formula of the hydrated
compound is CoCl2⋅6H2O, which is cobalt(II) chloride hexahydrate.
Let’s summarize what we’ve learnt
about volatilization gravimetry. We learnt that volatilization
gravimetry involves separating volatile compounds from a sample and measuring the
change in mass. This quantitative analytical method
can be used to determine the mass of the remaining nonvolatile substances and, from
this, the mass of the liberated volatile substances.
We looked at specific examples of
hydrated salts, where the remaining nonvolatile substance is an anhydrous salt and
the liberated volatile substance is water vapor, which comes from the water of
crystallization in the hydrated salt. And these mass values can be used
to determine the number of molecules of water of crystallization in the hydrated
salt formula. We also briefly learnt that
volatilization gravimetry can be used to determine the percentage of water of
crystallization in a hydrated salt.
