Magnesium hydroxide and magnesium
carbonate both react with sulfuric acid to produce magnesium sulfate. Describe how magnesium sulfate
crystals could be prepared from magnesium hydroxide and dilute sulfuric acid.
The first thing to note in this
question is that you’re required to make crystals, which suggests that, at some
point in the process, a crystallization is required. The second thing to notice is that
we are using dilute sulfuric acid. So it’s likely that magnesium
hydroxide will be in excess. This is okay because magnesium
hydroxide isn’t very soluble. So there are purification steps
that we can follow to remove it.
We can break down our answer into
two steps: the first reacting components together and the second purifying the
product so that we end up with magnesium sulfate crystals.
So the first step is that excess
magnesium hydroxide is stirred in dilute sulfuric acid. This will produce a solution of
magnesium sulfate with a suspension of magnesium hydroxide, which needs to be
removed. The easiest way of removing a
suspension is by filtration.
So after the reaction is complete,
the product suspension is filtered to remove excess magnesium hydroxide. After filtration, it’s likely that
the magnesium sulfate solution won’t be very concentrated. So we wouldn’t get good crystals if
we crystallize it straight away.
So the first thing to do is remove
some of the excess water. This is done by heating the
filtrate in an evaporation disc using a Bunsen burner. Once enough of the liquid has been
removed that the mixture has started to crystallize, the mixture is then cooled
using an ice bath so that crystals form. So the filtrate is heated to
evaporate most of the water, then cooled until crystals form. And that’s how you would generate
magnesium sulfate crystals from magnesium hydroxide and dilute sulfuric acid.
The equation for the reaction of
magnesium hydroxide with sulfuric acid is Mg(OH)2 + H2SO4 react to form MgSO4 +
2H2O. The relative atomic masses in grams
per mole of hydrogen is 1.0, of oxygen is 16.0, of sulfur is 32.1, and of magnesium
is 24.3. A student reacts magnesium
hydroxide with excess sulfuric acid and calculates that the maximum yield of
magnesium sulfate is 6.40 grams. Calculate the mass of magnesium
The first thing to notice here is
that the question says there’s an excess of sulfuric acid. So when magnesium hydroxide reacts,
all of it should be consumed. And the yield of magnesium sulfate
should be 100 percent.
So the process we’re going to go
through is we’re gonna take the mass, 100 percent yield of magnesium sulfate, and
divide it by the molar mass of magnesium sulfate. This will give us the number of
moles of magnesium sulfate.
We know from the chemical equation
that the ratio of magnesium sulfate to magnesium hydroxide is one to one. Therefore, we’ll know that the
number of moles of magnesium hydroxide is the same as the number of moles of
magnesium sulfate. So all we need to do is multiply
through by the molar mass of magnesium hydroxide to get the mass of magnesium
hydroxide. This leaves two unknowns.
What are the molar masses of
magnesium sulfate and magnesium hydroxide? The molar mass is equal to the sum
of the atomic masses of the elements involved. For magnesium sulfate, that means
one equivalent of magnesium, one equivalent of sulfur, and four equivalents of
oxygen, which means it’s 24.3 plus 32.1 plus four times 16. This is equal to 120.4 grams per
For magnesium hydroxide, we have
one equivalent of magnesium plus two equivalents of oxygen plus two equivalents of
hydrogen, which is equal to 24.3 plus two times 16 plus two times one. This is equal to 58.3 grams per
Going back to our calculation,
we’re now going to divide 6.40 grams of magnesium sulfate by the molar mass of
magnesium sulfate, which is 120.4. This is equal to 0.053156 moles
approximately. The number of moles of magnesium
sulfate produced is the same as the number of moles of magnesium hydroxide
reacted. So the number of moles of magnesium
hydroxide, 0.053156, is multiplied by the molar mass of magnesium hydroxide,
58.3. This gives a mass of 3.0990 grams
approximately. The least significant value in our
question is stated to three significant figures. Therefore, our answer should be to
three significant figures, which means our answer is 3.10 grams.
Now I’m just going to summarize the
results from this section. So from that part, we have the
molar masses of magnesium hydroxide and magnesium sulfate and 100 percent yield of
6.40 grams magnesium sulfate.
The percentage yield of the
reaction is found to be 83.0 percent. Calculate the mass of magnesium
To answer this question, we can go
back to the definition of percentage yield. Percentage yield is equal to the
actual mass produced divided by the theoretical maximum mass possible. If we rearrange this equation, the
actual mass is the theoretical maximum mass multiplied by the percentage yield. This is equal to 6.4 grams, which
is the maximum yield, 100 percent, from the previous question, multiplied by 83
percent, which is the same as 83 over 100. This is equal to 5.312 grams
approximately. Given that the least significant
value in our question is given to three significant figures, our answer should be to
three significant figures. So our answer is 5.31 grams.
The equation for the reaction of
magnesium carbonate with sulfuric acid is MgCO3 + H2SO4 react to form MgSO4 + CO2 +
H2O. The relative formula masses of some
of the components are magnesium carbonate 84.3, sulfuric acid 98.1, carbon dioxide
44.0, and water, H2O, 18.0. The atom economy of a reaction is
calculated using the equation atom economy is equal to the relative formula mass of
the desired product divided by the sum of the formula masses of all the reactants
multiplied by 100 percent. Calculate the atom economy for the
production of magnesium sulfate from magnesium carbonate. Give your answer to three
I’ve just cleaned that up and
borrowed the fact that we have the molar mass of magnesium sulfate from the previous
question and reexpressed the question thus. What is the atom economy for the
production of magnesium sulfate from magnesium carbonate?
So the atom economy is equal to the
relative formula mass of the desired product divided by the sum of the formula
masses of all reactants multiplied by 100 percent. Our desired product is magnesium
sulfate with a molar mass of 120.4. Our reactants are magnesium
carbonate and sulfuric acid, with molar masses of 84.3 and 98.1, respectively. This is equal to 0.6601
approximately multiplied by 100 percent, which is the same as 66.01 percent. The question asks for our answer to
three significant figures, so our answer is 66.0 percent. I’ll pop that value up there for
When magnesium sulfate is produced
from magnesium hydroxide, the atom economy is 77.0 percent. Explain why this atom economy
differs from the value obtained when magnesium sulfate is produced from magnesium
The first thing to notice is that
the atom economy when magnesium sulfate is produced from magnesium carbonate is less
than when it’s produced from magnesium hydroxide. This could be understood by going
back to the definition of atom economy, which in this case is equal to the molar
mass of magnesium sulfate divided by the sum of the molar masses of the
reactants. In both cases, the magnesium
compound is reacting with sulfuric acid and only one equivalent thereof.
Therefore, any difference in the
atom economy arises because of the difference in the molar masses of the magnesium
compounds. The molar mass of magnesium
carbonate is greater than the molar mass of magnesium hydroxide, 84.3 being greater
than 58.3. This can be written out as the
reaction of magnesium carbonate has a lower atom economy because one mole of
magnesium carbonate has a greater mass than one mole of magnesium hydroxide but
reacts to produce the same amount of magnesium sulfate.