Video Transcript
A sample of magnesium sulfate
hydrate, MgSO4⋅𝑥H2O, is heated until its mass remains constant. For every mole of MgSO4, seven
moles of water are liberated. What is the value of 𝑥?
In this question, we have a sample
of magnesium sulfate hydrate. The word “hydrate” tell us that we
have a hydrated salt. A hydrated salt is a substance that
contains water of crystallization. Water of crystallization is the
presence of water molecules within the structure of a crystal. We can see from the chemical
formula that for every formula unit of magnesium sulfate in the crystal structure,
there are 𝑥 number of molecules of water associated with it. This is the water of
crystallization. And for many hydrated salts, the
value of 𝑥 can vary.
Our job in this question is to find
the coefficient in front of H2O in the chemical formula, which is represented by
𝑥. One way to determine the water of
crystallization is to use an experimental technique called volatilization
gravimetry. In this method, thermal or chemical
energy is used to separate substances so that their masses can be measured
independently. In this question, we are told that
the magnesium sulfate hydrate is heated until its mass remains constant. This could be achieved by heating a
sample of the hydrated salt in a crucible with a Bunsen burner. During heating, water molecules are
liberated from the hydrated salt. When all of the water has been
removed from the hydrated salt, the mass of the salt stops changing. And what’s left behind in the
crucible is called the anhydrous salt.
We know that for every one mole of
magnesium sulfate, seven moles of water are liberated. We stated earlier that the chemical
formula of the hydrated salt tells us how many water molecules are associated per
formula unit of magnesium sulfate. So we could also write this ratio
as one formula unit of magnesium sulfate per seven molecules of water. Therefore, the value of 𝑥 must be
seven, and the chemical formula of the magnesium sulfate hydrate is MgSO4⋅7H2O.
In conclusion, the value of 𝑥 in
the provided chemical formula of magnesium sulfate hydrate is seven.
