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 we'll 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 Avagadro'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 you 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 we can calculate different masses.

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 substances 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.

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 and 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 that mathematical relationship of mass and grammes equals the number of moles times the relative formula mass means that it's worth us taking a moment just to remind ourselves that the relative formula mass of a substance is equal to the mass of the relative atomic masses of all the atoms in that substances formula.

So we have an example here of sulphur dioxide SO2, and I can get some information that I need in order to calculate the relative formula mass straight from that formula.

It tells me that I have one atom of sulphur and two atoms of oxygen.

At this point then I can go to my periodic table and find out what the relative atomic mass is for each of those elements.

So my sulphur is worth 32.

1 and the oxygen is 16.

0.

What I need to do now then is multiply the number of atoms by the relative atomic mass of each.

So the sulphur then is bringing a total relative mass of 32.

1 to my entire molecule, and the oxygen then is bringing a relative mass of 32.

0 to the sulphur dioxide molecule.

The relative formula mass then for this particular substance of sulphur dioxide is simply an addition of all those and that brings me to 64.

1.

Now it's worth remembering that the number of particles in one mole of different substances is going to be exactly the same, 6.

02 times 10 to the 23, but the mass of those different substances that have one mole of particles is gonna be different.

And that's because both of those substances have different relative formula masses.

So I have here an example of one, sorry, not 0.

1 moles of sodium chloride and not 0.

1 moles of sulphur.

So in both of these samples I have the exact same number of particles, 6.

02 times 10 to the 22, but the relative formula mass for sodium chloride is 58.

5, whereas the relative formula mass for sulphur is 32.

1.

That means when I put these two samples onto a scale, if I want the exact same number of particles, the mass that I would actually measure out for them is going to be completely different, because they have a different relative mass per particle.

Let's look at how we can use that relationship to find the mass of not 0.

5 moles of water.

We remember that relationship is massing grammes is equal to the number of moles times the relative formula mass.

Now you've been given the moles of not 0.

5, but we need to find the relative formula mass from the formula, and when we do that, we get a value of 18.

0.

So if we take 18.

0 times not 0.

5, we get a value of 9.

0, which means 0.

5 moles of water will have a mass of 9.

0 grammes on a balance.

What I'd like you to do now is to calculate the mass of 3.

2 moles of butane.

So you'll need to get yourself a periodic table, a calculator and pause the video so you can do your working and come back when you're ready to check your answer.

Okay, let's see how you got on.

So we're using the same mathematical relationship and you've been given the moles of 3.

2.

The formula you need to find the relative mass for is C4H10, and when you do that you get a value of 22.

0.

So 3.

2 times 22.

0 gives you a value of 70.

4, which means 3.

2 moles of butane will have a mass of 70.

4 grammes.

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

I think it's really important here as well to remind you that you need to be showing you working out so that in case you make any errors, we can quickly and easily identify where they're being made so that we can adjust your strategy ever so slightly and try to avoid those errors in future, but very well done if you've managed to get that correct.

What a cracking start, guys, keep it up.

Let's have a go at another quick check.

What I'd like you to do is to calculate the mass of 1.

25 moles of calcium carbonate with the formula caCO3, and to give your answer to three significant figures.

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

Well done if you chose D, 125 grammes.

Now, if you weren't sure of the working out, I've shown that on the side here, if you got an answer closer to B or C, it's quite possible that you've put the atomic number rather than the relative atomic mass into your calculation for relative formula mass.

So be careful that you're using the correct values off your periodic table when making that calculation, but very well done if you manage to get D, 125 grammes.

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 reckon that the mole of oxygen is made up of one mole of O2 molecules and the relative molecular mass of O2 then is two 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 moles of oxygen atoms have a mass of two 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 beating 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 equation you needed was mass and grammes equals relative formula mass times moles, and the relative formula mass for sodium chloride is 58.

5.

When you multiply that by 1.

5, you get 87.

8 grammes.

What I'm gonna do for the rest of these answers then is just give you the mass in grammes for each of them.

And if you didn't get the correct answer, go back to your relative formula mass to see if that was correctly calculated as well.

So oxygen should be 48.

0, zinc is 98.

1, carbon dioxide is 66.

0, nitric acid is 94.

5, calcium bromide is 299.

9 and iron three hydroxide was 160.

2.

Very well done with this guys.

I'm really impressed 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 hoard 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 hoard? Well, what you'd probably do is just divide the mass of the entire hoard 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 hoard 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 substances relative formula mass.

So in order to find the number of particles or the number of moles a particular substance contains, all we need to do is rearrange that equation.

And we're gonna do that by dividing by relative formula mass on both sides of that equation.

And when we do that, we get a new equation that is moles equals the mass in grammes divided by the substances relative formula mass.

So if we want to know how many moles there are in 112 grammes of nitrogen, we've been given the mass and it's in grammes essential, the relative formula mass for nitrogen then is going to be 28.

0.

And when we divide the two, we get a value of four moles of nitrogen are found in 112 grammes.

So let's look at another example.

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

So I'm going to use that relationship of moles equals mass and grammes divided by the relative formula mass.

I've been given the mass of 45 grammes in the question and I need to find the relative formula mass for my substance, which is water.

And when I do that, I find it to be 18.

0.

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

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

What I'd like you to do now then is to calculate how many moles are present in a 30 gramme sample of butane.

So you're gonna need your calculator, a periodic table, and to pause the video and come back when you're ready to check your answer.

Okay, let's see how you got on.

So you're going to use that same relationship.

Moles equals mass and grammes divided by relative formula mass.

You've been given the mass of 30 grammes and the relative formula mass needs to be found for butane with the formula C four H 10, and that is 22.

0.

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

36 moles of butane can be found in a 30 gramme sample.

Very well done if you manage to get that correct, hopefully you're still showing you're working out so we can identify any errors if they're happening and correct them going forward, but I'm so pleased with the work you guys are doing today.

Fantastic job, keep it up.

Let's stop here for another quick check.

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

Well done if you said C, this is the answer correctly rounded from our value of 9.

8749 that you get when you are dividing the mass by the relative formula mass.

Well done if you chose C.

Now we said earlier that different particles will have different relative formula masses, because of the atoms that make up that substance.

And if we have the same number of particles of two different substances, they would have different masses because of that, but it works in the other way as well.

If you have the same mass of different substances, they will contain a different number of moles, a different number of particles.

Now if we look an example here, I have five grammes of sugar and five grammes of copper sulphate.

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

6.

If I calculate the number of moles for each of these, I can see that there are significantly more particles in a five gramme sample of copper sulphate than there are in a five gramme sample of sugar.

And the reason is because it takes fewer particles of a large relative formula mass sample to make up a particular mass in grammes than a substance that has a smaller relative formula mass.

So if you have large particles, you need fewer of them to get to a particular mass and if you have a smaller mass particle, then it will take more particles to get to that same mass in grammes sample.

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 Avagadro's constant.

The number of particles in a sample is gonna be equal to the number of moles times Avagadro'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 not 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, not 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 mass and gramme sample of different substances.

And to complete this table, you may wish to use some of the data that you calculated from task eight part two 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.

So what we have here then, if you compared this task to that from task A part two is that all the formulas are the same.

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

And when you do, this is what you should have calculated for the moles, sodium chloride should be not 0.

342.

Oxygen is not 0.

313, zinc is not 0.

948.

Carbon dioxide 0.

0168.

Nitric acid is 0.

162, calcium bromide 1.

77 and iron hydroxide is 8.

85.

And what I've done with all of these answers is to give them two, three significant figures if you're wondering why there's a slight difference.

So well done if you've managed to calculate all those 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 to hand, what you can do is to click on the watch button to watch a video 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 tear 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 and 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 Avagadro'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.

So the first thing I wanna do before I go through my answers is just remind you that your answer is gonna depend on the mass of water that you calculated.

Now my calculations are gonna follow through from the practical video, but yours are gonna depend on the mass you actually calculated.

So what you're looking for in your own answers is the processing.

Have you multiplied or divided correctly? Have you used your numbers properly as we go through, the first thing you needed to do was to calculate the relative formula mass for water, which was 18.

0.

Then you needed to use that value and the mass of your water to find the number of moles of water that fit on the penny.

And for the practical video, it was 0.

080556.

Now that's a lot of significant figures.

Don't go crazy here, it's as long as you've got at least 0.

0806, you're on the right track.

We're going to then use that value to find out the number of water molecules that fit onto that penny head by multiplying the number of moles by Avagadro's constant.

And when we do that, we were able to fit a whopping 4.

85 times 10 to the 22 water molecules on the head of a penny.

I actually really, really like this practical.

It's one of those things you can bring all the maths together.

If you have access to other coins, you may want to try and estimate how many particles you can actually fit on that compared to the size of your penny and see how well you're able to do.

Okay, so there's lots of possibilities you can do on this.

See what you can do now that we have these mathematical relationships.

The world is your oyster on how many particles you could fit on things, but very well done, I hope you enjoyed being able to do a little bit of practical with the maths.

It's not something that we get a lot of chances to do, so I hope you enjoyed it.

I hope you guys had a good time today.

I really enjoyed that penny practical, and I hope you did too.

But let's just take a moment to summarise what we've managed to do in today's lesson.

First of all, we reminded ourselves that the number of particles in one mole of a substance doesn't change.

It stays at 6.

0, two times 10 to the 23rd, and that the mass of one mole of a substance is gonna be equal to its relative formula mass of the substance, but measured in grammes.

So we get this mathematical relationship of the mass in grammes is equal to the number of moles times the relative formula mass, and we can represent that mathematical relationship in a variety of ways.

But crucially, what we've learned, the crux of today's lesson is that a mathematical relationship in chemistry can be manipulated to process our practical results to help us calculate the number of moles or when we use Avagadro's constant, the number of particles that we actually have in a substance.

And these mathematical relationships form an arsenal of resources that we can use to calculate other things as we move through this unit and beyond and further in our journey through chemistry.

I had a really good time learning with you today.

I hope you enjoyed learning with me, and I hope to see you again soon, bye for now.