Hello, my name's Mrs. Nevin, and today we're going to be talking about moles and masses as part of our unit on calculations involving masses.

Now you may have some experience of this from your previous learning, but what we do in today's lesson will not only help us to better answer that big question of what are substances made of, but will also help us to understand a little bit more clearly the mathematical relationships that exist within chemistry and how we can exploit and manipulate those relationships to calculate an unknown.

So by the end of today's lesson, you should be able to calculate both the mass of a substance and the number of moles it might contain based on data that's been provided.

Now throughout the lesson I'll be referring to some key terms, and these include: relative formula mass, mole, and Avogadro's constant.

Now these definitions are provided on the next slide in sentence form, and you may wish to pause the video here to make a quick note of them so that you can refer to them later on your learning or later on in today's lesson.

So today's lesson is broken up into two parts.

We'll start by looking at how we can calculate different masses and then move on to look at how we can calculate moles.

So let's get started by looking at how different masses can be calculated.

Let's imagine for a moment that you have a gold bar and it has a mass of 11.

3 kilogrammes.

How might you go about calculating the mass of a dozen gold bars? Well, the first thing you need to remember is that the word dozen refers to the number 12.

So you have 12 gold bars.

Then what you'd need to do is multiply that number by the mass of one gold bar, which is 11.

3.

So 11.

3 times 12 gives you a value of 135.

6, which means a dozen gold bars would have a mass of 135.

6 kilogrammes.

Now we're not gonna be using any gold bars here, but what this does point out is a mathematical relationship that chemists can also use in order to find the mass of multiple particles.

In practical settings, chemists are able to calculate the mass of a substance by using a balanced symbol equation, and it starts with the coefficients that are available in this balance symbol equation, telling us the ratio of the particles in our reactants and our products.

So from this particular reaction, we can say that we have two moles of magnesium atoms, one mole of oxygen molecules, and two moles of formula units that are composed of one magnesium ion and one oxide ion.

Now the mass then of these different substances is gonna be related to the number of particles that are present and the substance's relative formula mass.

Now the amount of chemicals that are present in a sample tends to be measured in moles.

Okay, so we're talking about the number of particles are gonna be measured in moles.

Now, chemists use moles to describe the quantities in a reaction for a variety of reasons.

For one reason, it is a lot simpler to talk about one mole of a substance, a package of 6.

02 times 10 to the 23 particles rather than that number of particles every single time.

So the numbers are a little bit simpler.

Secondly, it's far more accurate to talk about moles because we can talk about a large number of particles, be them atoms, molecules or ions without having to count them up individually.

They're just far too small for us to be able to do that.

So we're keeping the numbers simple while still being accurate in our descriptions.

And the most important thing with moles is that it tends to link up the fundamental concepts of chemistry.

Now, the fundamental concepts in chemistry are threefold.

I like this triangle because they are interconnected and it's one of the reasons that can sometimes make chemistry seem a little bit tricky.

But the three main aspects that we're talking about here is first of all, the macroscopic, the practical work, the stuff that you can actually see with your own eyes that's taking place.

The next concept in chemistry that moles helps to link is this symbolic idea, how we represent those chemical reactions that we can see, and we do that using our chemical equations.

And then finally, we have what's known as the microscopic, that's that theoretical idea of what's going on with these particles.

So we're talking about our individual atoms, our molecules and our formula units and moles helps us to link up all these different ideas in one fell swoop.

So how does it work? Well, in order for us to link up those different concepts, the thing that we start with is our balanced symbol equation, because what it's doing is it's representing that chemical reaction that we see within that lab, anyway.

So we know that we're taking a strip of magnesium reacting with some oxygen in the air and making this magnesium oxide powder.

But, significantly, in this representation of that chemical reaction, we have coefficients and they can actually be interpreted in two different ways.

Okay? So we can interpret them on the small scale, looking at them in terms of individual particles, so the magnesium atoms, the oxygen molecule or the formula units that we have in our magnesium oxide.

But we can also use these coefficients to talk about this reaction on a large scale using this idea of moles.

So these packages of 6.

02 times 10 to the 23 particles.

So looking at this reaction, again, I can actually read it as saying I have two moles of magnesium atoms that will react with one mole of oxygen molecules to form two moles of formula units of my magnesium oxide.

So the number of moles will help indicate the number of particles that are present in all of the substances of my chemical reaction.

And the relative formula mass then will indicate the mass of each individual particle.

Taking that into consideration, then I have this mathematical relationship that the mass in grammes will be equal to then the number of moles, the number of particles present times the relative formula mass, the mass of each individual particle.

Now the mass then, in grammes, will be measured using a balance, and it's crucial when I use this mathematical relationship that we're talking about mass in grammes, the relative formula mass then is something that I can calculate using the substances chemical formula and a periodic table.

That means I don't need to actually count up individual particles to find the number of moles, but I can calculate the number of moles using this mathematical relationship.

So really this equation is the first one in our arsenal of equations that we as chemists can use to help us find an unknown value.

And how we then manipulate this as we go forward will help us to find any number of unknown values, and we're gonna take it nice and slow focusing in first of all on how we can calculate masses.

Now sadly, in chemistry, the only kind of cheat sheet that you're allowed when you take an assessment is your periodic table.

So these equations don't tend to be provided.

So it's helpful to come up with ways for you to be able to remember these relationships as you go forward.

Now the equation as it stands, mass in grammes equals moles times the relative formula mass is kind of lengthy.

So one of the things we could do to help us remember it is to reduce down what we're writing.

So changing relative formula mass to RFM.

Now you might see RFM actually refer to as relative molecular mass or relative formula mass.

I personally like this, the Mr, it's even less to write and relative mass helps me to remember exactly what I'm supposed to be calculating here in order to use it in my equation.

You could actually rearrange this last equation to one that might be a little bit easier to remember is mass equals Mr or Mr. Moles, Mr. Moles.

Now one of the most essential parts of using that equation of mass equals moles times relative formula mass is being able to actually have or calculate that relative formula mass.

And so it's worth taking a moment here to just remind ourselves how that's actually calculated.

And a relative formula mass then is simply the sum of all the atoms that are in that substances formula.

So if we look at an example here, I have sulphur dioxide.

Now the element that is involved and the number of atoms is provided directly from the formula.

So I could actually fill in this little table using that formula.

So I have one sulphur atom and two oxygen atoms. At this point then I'm gonna have to use my periodic table.

So I move to that and I can see then the relative atomic mass for my sulphur is gonna be 32 and the relative atomic mass for my oxygen is 16.

I have all the information I need now I just need to do a little bit of maths to find that relative formula mass.

What I'm going to do is to multiply the number of atoms for each element by the relative atomic mass for that atom.

So I get a total of 32 for my sulphur and 32 for my oxygen.

So that's the relative mass that's being brought to this particular substance.

Then all you need to do is add up the total for each of your elements.

And when you do that, you have your relative formula mass for that particular substance.

Now it's worth remembering that the number of particles in one mole of a substance stays the same, 6.

02 times 10 to the 23.

But the mass in grammes of each of those substances of one mole will be different.

And that's because each substance has that different relative formula mass, okay? So I have an example here of 0.

1 moles of sodium chloride and sulphur.

Now in each of these samples we have the exact same number of particles, 6.

02 times 10 to the 22.

But the relative formula mass for my sodium chloride is 58.

5 and the relative formula mass for my sulphur is 32.

That means then that the mass that I would actually measure out on a balance in the lab for my sodium chloride is gonna be different than the mass that I would measure out for my sulphur, even though there's the same number of particles because they have a different mass per particle.

So let's look at how I can use that relationship to find the mass of 0.

5 moles of water.

So my mathematical relationship is mass in grammes equals moles times relative formula mass.

Now I've been given the moles in the question of 0.

5, but I need to find the relative formula mass.

So I need to find the chemical formula H2O, and when I go through the processing there, I find the relative formula mass is 18.

So if I take 0.

5 times 18, I get a value of 9, and that tells me then that 0.

5 moles of water will have a mass of 9 grammes.

So I've gone through an example here.

What I'd like you to do now is to have a go at calculating the mass of 3.

2 moles of butane.

So at this point you'll need to make sure you have a periodic table and a calculator handy.

You're probably gonna wanna pause the video here then and come back when you're ready to check your answer.

Okay, let's see how you got on.

So we have the same mathematical relationship and we've been given the number of moles of 3.

2.

Our formula that we need to find the relative mass for is C4H10, and that should give you a value of 22.

When you multiply then 3.

2 times 22, we get a value of 70.

4, which means that 3.

2 moles of butane has a mass of 70.

4 grammes.

Very well done if you manage to get that correct.

Now I just wanna remind you how important it is that you are writing your working out so that if you do go wrong somewhere, we can quickly and easily find out where that error is and correct it as we go forward.

So very, very well done though if you manage to get that correct.

Great start guys.

Let's have a go at another quick check.

What is the mass of 1.

2 moles of calcium carbonate the formula CaCO3? pause the video and come back when you're ready to check your answer.

Well done if you chose d, the answer should be 125 grammes.

And the working out is shown here for you.

If you got a value for B or C, it's possible that you actually use the atomic number rather than the relative atomic mass number for your calculation of the relative formula mass.

So make sure you're using the correct number of your period table when you are calculating relative formula mass.

But well done if you got D.

Okay, let's move on to the first task of today's lesson.

So we have some students who are discussing the best way to calculate the mass of one mole of oxygen molecules, and I'd like you to decide who you agree with and why.

So I'm looking for a because clause in your answer.

So Laura reckons that one mole of oxygen has a mass of one times the relative atomic mass of oxygen in grammes.

And because the relative atomic mass of oxygen is 16, she reckons the mass of one mole of oxygen should be 16 grammes.

Alex, on the other hand, thinks that one mole of a substance has the mass of 12 grammes like carbon, so the mass of one mole of oxygen should be 12 grammes.

Lucas reckons that the mole of oxygen is made up of one mole of O2 molecules and the relative molecular mass of O2 then is 2 times 16 is 32.

And so the mass of one mole of oxygen should be 32 grammes.

Izzy thinks that a mole of O2 molecules are made up of two moles of oxygen atoms. So two mos of oxygen atoms have a mass of 2 times 16, and that's 32 grammes.

Who do you agree with the most and why? Pause the video here and come back when you're ready to check your choices.

Okay, let's see how you got on.

Now, Laura was on the right track when she thought that the relative atomic mass of oxygen is related to the mass of a mole, but what she hasn't realised is that oxygen is diatomic, its formula is O2, and so the mass of the mole is not 16 grammes.

We have to talk about the entire formula, which is two atoms of oxygen.

So she's not on the right track.

Alex has also realised that it's a different substance and therefore it has a different relative formula mass.

So if we have the same number of particles because it's a different relative atomic mass, it should have a different mass than carbon.

So he is also not on the right track.

Now Izzy was actually on the right track.

She's calculated the correct answer, but the way she's done it is incorrect because when we're talking about a mole, we're talking about counting up particles.

And the fundamental particle for oxygen being a diatomic molecule is the molecules of oxygen, not the individual atoms. So rather than looking and calculating two moles, we wanted to know the mass of one mole of oxygen.

So we need to be talking about it in terms of the molecules, not the individual atoms. So she was close, but there was a better answer.

Lucas has correctly identified that a mole of oxygen is formed of O2 molecules and therefore the relative formula mass for oxygen is gonna be 32 and one mole then is equal to the relative formula mass for that molecule in grammes of 32 grammes.

So well done if you chose either Izzy or Lucas.

Lucas was pipped Izzy to the post because he's been able to talk about moles in a more accurate manner, talking about the molecule particles.

So very well done on a tricky first task.

Good job guys.

For the second part of this task, what I'd like you to do is to complete the table below.

You've been given the substances and their formula.

You need to find the relative formula mass and then the mass for 1.

5 moles in grammes of each of those substances.

So again calculators at the ready, periodic table handy, pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

So the relative formula mass for sodium chloride was 58.

5, and when we multiply it by the number of moles, we get 87.

8.

What I'm gonna do now is go through the answers here and I'll give you the mass.

Okay, so the mass of oxygen should be 48 and to double check you have the correct relative formula mass in case you have the mass in grammes incorrect, you have all the information here you needed.

So the zinc should be 97.

5 grammes, carbon dioxide is 66 grammes, nitric acid is 94 grammes, calcium bromide was 300 grammes, and iron III hydroxide was 160.

5 grammes.

So very well done if you managed to get those masses correct.

If all of those are correct, then you're working out, it's probably correct as well.

And I'm really pleased with the start you've made to this lesson.

Keep it up.

Now that we're feeling comfortable calculating masses, let's look at how we can calculate moles.

Let's imagine you found a horde of coins.

And it had a mass of 955.

4 grammes, but each coin had a mass of 1.

7 grammes.

How might you calculate how many coins are actually found in that horde? Well, what you'd probably do is just divide the mass of the entire horde by the mass of one coin.

And when we do that, we get a value of 562, which would tell us that there are about 562 coins in the horde of coins.

Now, chemists use a really similar process when they're trying to find out the number of particles i.

e.

the number of moles that a sample contains using that mathematical relationship of mass in grammes is equal to the number of moles times a substance's relative formula mass.

So in order to find the number of moles and the number of particles, all we need to do is rearrange that equation to make moles the subject of the equation.

And in order to do that, what we need to do is to define both sides of our equation by the relative formula mass.

And when we do that, we get that new equation of moles is equal to the mass in grammes divided by relative formula mass.

So if we wanted to look at how many moles 112 grammes of nitrogen contains, we'd have our mass in grammes, which is essential.

And then we have our relative formula mass for nitrogen is going to be 28.

So when we divide those two values, we could say that four moles of nitrogen are found in 112 grammes.

Let's look at another example.

I'd like to know how many moles there are in 45 grammes of water.

So again, I'm gonna use that equation of moles equals mass in grammes divided by relative formula mass.

And then I need to find those two pieces of information.

Now the mass has been given to me in my question and I need to find the relative formula mass for my water, which I find to be 18.

When I divide those two values then, I get an answer of 2.

5 moles of water can be found in a 45 gramme sample.

What I'd like you to do now then is to calculate the number of moles that are present in 30 grammes of butane.

So you'll need your calculator, your periodic table, and then pause the video when you're ready to check your answer.

Okay, let's see how you got on.

So you're going to use that same mathematical relationship of moles equals the mass in grammes divided by the relative formula mass.

You've been given the mass of 30 grammes and the relative formula mass needs to be found for butane, which is C4H10, and that is 22.

When you divide those two values, then you should get an answer of 1.

36 moles of butane are found in a 30 gramme sample.

So very well done if you manage to get that correct.

Hopefully you are still showing your working so that we can find any errors and correct them going forward.

But I'm really pleased with the work that you guys are doing today.

Fantastic job.

Let's have another quick check.

How many moles can be found in a 1,500 gramme sample of iron sulphate? Pause the video here and come back when you're ready to check your answer.

Well done if you chose C, 9.

9 moles.

If we go through the working out, you should have had an answer of 9.

8684, but when it's correctly rounded, it should be 9.

9 moles as your final answer.

Well done if you got that correct.

Now, we said earlier that different particles will have different relative formula masses and that if you have the same number of particles because they have different relative masses, the mass on a balance would be different for one mole, but it works the other way as well.

If you have the same mass in grammes of different substances, then those samples will contain a different number of particles.

So I have an example here of five grammes of sugar and five grammes of copper sulphate.

And the relative formula mass for sugar is 342 whilst for copper sulphate it's 159.

5.

So copper sulphate is significantly lower in its relative formula mass than the sugar is.

And when I find out the number of moles or the number of particles in that five-gram sample, the sugar contains 0.

9146 moles of particles, whereas the copper sulphate contains significantly more.

And that's because substances with a larger relative formula mass will contain fewer particles in the same mass as another substance that has a smaller relative formula mass.

You need fewer particles of a heavier particle to make up a particular mass.

That's essentially what that means.

Let's stop here for a quick check.

If I have two samples, 12 grammes of carbon and 12 grammes of sulphur, how do the number of atoms in each compare to each other? Well done if you said a, there's going to be more carbon atoms in that 12 gramme sample because it has a lower relative formula mass than the sulphur does.

So there'll be more carbon atoms in its 12 gramme sample than there are sulphur atoms in its 12 gramme sample.

Now, lastly, I just wanna remind ourselves about how we can use this number of moles to be more specific about the number of particles that are in a sample.

And we come back to this idea of using Avogadro's constant.

The number of particles in a sample is gonna be equal to the number of moles times Avogadro's constant.

So in this example of having 2.

2 moles of calcium carbonate, I actually have 1.

32 times 10 to the 24 particles of calcium carbonate.

If I had only 0.

25 moles of that calcium carbonate sample, I would actually have 1.

51 times 10 to the 23 particles.

So this idea of moles, again, is a really easy way of being able to talk about having 2.

2, 0.

25.

But these mathematical relationships allows me to be really specific about the number of particles that I'm actually talking about in a different sample.

Okay, let's move on to the last task of today's lesson.

What I'd like you guys to do, first of all, is to calculate the number of moles in a different massing gramme sample of different substances.

And to complete this table, you may wish to use some of the data that you calculated from Task A, part 2 to help you get started on this, but you will definitely need a calculator and possibly a periodic table as well.

So pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

Now you may have noticed if you compared this task to that from Task A, part 2 that the formulas were all the same.

So you could have just copied the relative formula masses from that task into this one.

But the moles then should be this.

Sodium chloride should be 0.

0342.

Oxygen is 0.

313, zinc is 0.

954, carbon dioxide is 0.

0168, the nitric acid is 0.

162, calcium bromide is 1.

77, and the iron hydroxide then is 8.

83.

Well done if you managed to calculate those all correctly.

Okay, for the next part of today's task, I'd like us to do a bit of a practical to bring together what we've been learning.

So what I'd like you to do is to follow the method below to complete the table first of all.

Now this might take a little bit of practise, so you may wish to figure out what's the best way to put the paper towel on your balance.

Maybe you'd like to use a filter paper instead, but definitely practise the control and see how many drops of water you can fit on the head of a penny.

Now you might wanna do heads up, you might wanna do tails up.

It's completely up to you which you would like to use, but the key here is to keep track of the mass and how many drops that you have managed to fit of water onto your penny without it spilling over.

If it spills over, you've got to start again.

So the key here is to keep track of that mass.

I might recommend one person keeping track of the display on the balance whilst somebody else is actually dropping the water on the penny.

Now if you don't have any of this resources at hand, what you can do alternatively is to click on the watch button to watch a video of of it as well.

So the first thing we do is put a piece of paper down to collect any water that may spill over and tare or zero the scale, so it says 0.

00 grammes and then when the penny is put on, we can see the mass of the penny is 3.

53 grammes.

Once we start to drop water onto that penny, we can see the mass is starting to increase as that water starts to collect, the more and more that we put on top of it.

So keep adding drops and we're now nearly over four grammes now collected between the water and the penny, we can see it's starting to bulge a little bit due to the surface tension on that water.

Now if we zoom in a little bit, we can see we've got quite the bubble of water at 4.

61 grammes.

Now our penny and water are, and we're adding a few more drops.

You can see it's starting to go up quite significantly.

With each drop we can see that bubble of water is bulging ever so slightly.

So we know we're getting close to the maximum amount that can actually be balanced on this head of a penny.

Very, very close now.

One more drop and it's gone over.

So the last mass that I saw before it actually went over was 4.

98 grammes.

So once you've done this practical, you will have three different masses that you should have put into your table.

Now the measurements that I'm gonna show in the table here were taken from the practical video.

Yours may be different if you were able to carry this out yourself, but the mass of the dry penny from the video was 3.

53 grammes.

Once the water was put onto it, the last mass before it started to spill over was 4.

98 grammes, which means we were able to balance a whopping 1.

45 grammes of water on the head of a penny.

Now you might be wondering why you were asked to find out how much water you could balance, and the reason is so that we can process that information using your understanding of moles and particles.

So for each of these next three parts, you're gonna do some calculations and I'd like you to show your working and give your answers to three significant figures.

So you need to find the relative formula mass of water, calculate the number of moles of water that fit on that penny, and then I also want you to calculate the number of water molecules that were balanced on that penny.

And I've reminded you of what Avogadro's constant is here to help you on your way.

So pause the video and then come back when you're ready to check your calculations.

Okay, let's see how you got on.

Now, before I go through the answers for my calculations, I just wanna remind you that the answers are gonna vary depending on the mass water that you were actually able to fit on that penny and balance on it.

So my calculations follow the values that were provided from the practical video.

What you need to be doing if you have your own mass is to double check the processing.

So did you multiply or divide properly? Were you using the values in the correct places? That's what you're looking for in your calculation.

The first thing we needed to do was to find the relative formula mass for water and that was 18.

Then you needed to use that to find the number of moles of water that fit on the penny head.

So you're gonna take the mass of the water here for the practical was 1.

45 grammes and divide it by the relative formula mass and we get 0.

080556 moles.

Now there's a lot of significant figures there.

There are five significant figures and the reason is we never ever round our answers until the very final calculation.

Now this one is a bit excessive.

It could have been 0.

0806, but regardless we've got that value.

I need it though because that tells me the number of moles and I wanna know the number of water molecules that fit.

So I'm gonna take that value and multiply it by Avogadro's constant.

And when I do that, I was able to balance 4.

85 times 10 to the 22, a whopping, whopping amount of particles of water on that penny.

That was a lot of fun, actually.

I quite like doing the math and I hope you enjoyed it.

What you might wanna do is if you have a scale at home and you've got a dropping pipet, you could see if you could estimate about how many water molecules you might be able to fit on, say, something like a 10p piece or a 2p piece, would it make a difference if you used a pound coin or a two pound coin? If there's one that has little ridges on the side, would that help keep the water on? There's so many different things that you could play around with.

So if you're fancy doing that at home, by all means have a go.

So we've had a lot of fun today.

I hope you did anyway, doing that penny practical with the water.

But let's take a moment to at least summarise what we've learned in today's lesson.

What we've reminded ourselves, first of all is that one mole of a substance always contains the same number of particles.

That's 6.

02 times 10 to the 23rd.

And that the mass of one mole of a substance is gonna be equal to the relative formula mass of that substance measured in grammes when we're talking about just one mole.

But we can actually create a mathematical relationship, which is the mass in grammes is equal to the number of moles times a relative formula mass for that substance.

And it's sometimes written slightly differently, whereas relative formula mass might be looking like Mr Which is relative molecular mass.

Now, these mathematical relationships though, and this is the crux of today's lesson, is that the mathematical relationships that we have in chemistry can be manipulated to process results and calculate the number of moles or using Avogadro's constant, even the number of particles in a substance.

So it forms the start of our arsenal of mathematical relationships that we can use going forward in this unit and beyond.

And I really hope you enjoyed learning with me today.

I had a great time learning with you, and I'll see you again soon.