Video: GCSE Chemistry Higher Tier Pack 1 • Paper 1 • Question 5

GCSE Chemistry Higher Tier Pack 1 • Paper 1 • Question 5

10:18

Video Transcript

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 hydroxide reacted.

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 mol.

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 mol.

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 sulfate produced.

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 significant figures.

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 later.

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 carbonate.

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.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.