Hello, my name's Mrs. Niven, and today we're going to be talking about concentration of solutions as part of our unit on making salts.

Now, you may have some experience of what we talk about today from your previous learning, but what we do in today's lesson will help us to better answer those big questions of what are substances made of, how can they be made, and then how can they be changed, but also for us to better appreciate and understand some simple solutions that we come across nearly every day.

By the end of today's lesson then, you should hopefully feel more confident being able to explain what is meant when we talk about the concentration of a solution, as well as calculate the concentration of various solutions.

Now, the keywords that we'll be using throughout the lesson are given on the next slide, as well as their definitions.

You may wish to pause the video here so you can jot those down for reference later on in the lesson or later on in your learning.

Today's lesson then is broken into two parts.

We'll start by looking at describing what we mean when we use the word concentration, before we move on to discuss how it can be calculated.

So let's get started by looking at what we mean by concentration.

Many chemicals that chemists come across and use are found in their aqueous form, and these are indicated to us using that state symbol of aq, which you may recall simply means that a substance has been dissolved in water and exists as a solution.

So we have an example here of sodium chloride that has dissolved in water.

Then we have copper sulphate.

And many of the other chemicals that we commonly use in the science lab are also found in that aqueous form as a solution, things like acids.

Here we have sulfuric acid.

And our alkalis, here sodium hydroxide.

And we can tell that all of these are a solution simply by looking at their labels because they have that aq state symbol.

Now, a solution is composed of a solute that's dissolved in a solvent.

Now the solvent in an aqueous solution is simply water, H2O.

So the solute is our solid here, the solvent is our liquid that it's dissolving into, and the resultant mixture then is a solution.

Let's stop here for a quick check.

Which of the following statements is true about an aqueous solution of sugar? Well done if you said a and d.

Because it's an aqueous solution, we know that water is the solvent, and we'd be able to show that it's a solution using that state symbol of aq.

So very well done if you got at least one of those correct, and extremely good start to this lesson if you managed to choose both of those correct answers.

Fab work, guys.

Keep it up.

Now the thing about using the term concentration is that it actually has been used to explain a few different things in everyday life.

For instance, you might describe somebody focusing on their work as concentrating or having good concentration on that work.

The word concentration has even been used to describe World War II camps, and it might even be used to describe drinks as a concentrated drink.

But in chemistry specifically, concentration is used to describe a solution in terms of its solute and its solvent.

Concentration is simply a ratio of solute particles to the volume of solvent that those particles are dissolved in.

Now you may recall that volume represents the amount of space that a substance occupies.

So if we wanted to find the volume of this cube, we'd simply multiply its length times its width times its height, and we could say that the volume of this particular space is 1000 centimetres cubed, or one decimeter cubed.

So if I had two solutions of the same volume, concentration can help to indicate how crowded those solute particles are when they're dissolved in that solvent.

For instance, if I had one decimeter cubed of solvent and I only had a few particles of solute dissolved in it, I could describe the concentration of that solution as being low.

Few particles of solute, quite a low concentration.

If I had more particles dissolved in the same volume of solvent, so, many particles, I would describe that concentration as being high or higher than the other one.

So, many particles dissolved in a solvent gives us a high concentration.

And we can see that without having to zoom in to those solutions because higher concentration solutions tend to appear quite dark.

Now, I can control or adjust the concentration of a solution by changing the amount of solute that is dissolved in a specific volume of solvent.

So if I add a large amount of solute into a specific volume of my solvent, I'm putting many solute particles dissolving in that volume and I get a higher concentration.

If I want a lower concentration, I'm going to add a smaller amount of solute, providing fewer solute particles, and that would then result in that lower concentration.

Another way that I might be able to adjust the concentration of a solution is by adding distilled water to that aqueous solution, because what that does is it increases the volume of the solvent, so increases the space in which those solute particles are dissolved in.

Now what that actually is doing is diluting that original solution.

So a diluted solution has a lower concentration than the original solution.

So if my original solution has a high concentration of solute particles dissolved in a specific volume of solvent and I add some water to it, I end up with a larger total volume of solution.

But those solute particles are now spread out within that larger volume, creating a lower concentration.

Now it's really important if you are diluting a solution that you use distilled water because what that does is ensure that there are no other substances present in that solution.

You still have the same solvent with the same solute particles.

Let's take a moment for another quick check.

True or false? Solution X has a lower concentration than solution Y.

And I'd like you to explain your answer.

So justify your choice on whether or not that statement is true or false.

Well done if you chose false.

Now, there were a few different ways that you could justify that answer.

You could have said that solution X is darker in colour, suggesting that it contains more solute particles and therefore a higher concentration.

Or you could have referred to solution Y, saying that that is lighter in colour, suggesting that it contains fewer solute particles and therefore a lower concentration.

So very well done if you managed to choose the correct answer of it being a false statement, but incredibly well done if you were able to justify your choice with your explanation.

Great job, guys.

Well done.

Okay, time for the first task in today's lesson.

What I'd like you to do is use words from the box to complete the passages below.

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

Okay, let's see how you got on.

Now, if you completed the passages correctly, it should read out like this.

If a substance dissolves in water, it is said to be soluble.

The substance that dissolves is known as the solute, and the water is the solvent.

The resulting mixture is known as a solution.

Chemists often carry out reactions in solution.

As such, the concentration of each solution used should be stated.

This informs others about how much substance has dissolved in a certain volume of solvent.

Well done if you managed to get that correct.

If you used the incorrect word at any point, please do make sure that you have corrected that as we went through this so that you have a complete summary of what we mean by concentration.

Well done though if you managed to get those correct, guys.

Great job.

For this next part, I'd like you to use your understanding of concentration to help Jacob.

He's been asked to create a sweet cordial drink for his grandmother, but she thinks it tastes a little too sweet.

So what could Jacob do to change the concentration of his grandmother's drink so that it tastes less sweet? And I'd like you to explain how your suggestion would help him.

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

Okay, let's see how you got on.

Now, the best way to do this would be to simply add some water to his grandmother's drink because that would dilute the drink, making it less concentrated and that would help to make that drink taste less sweet.

We really don't want to be creating a whole new drink by pouring out what he's already made and then simply putting in less of that cordial to create that drink, which you could have done because that's just wasteful.

So if you did say start over, use less of the cordial drink and add some more water, then that would still be a correct answer.

But the better answer would be to use the drink you started with and simply add some water to it.

Well done though if you managed to suggest to add water, and very well done if you were able to explain how that would fix his grandmother's drink.

Very well done, guys.

Great start to this lesson.

Now that we're feeling a little more confident being able to describe and explain what we mean when we use that term concentration, let's move on to looking at how we can quantify it by calculating concentration.

Firstly, the volume of a solution is equal to the volume of the solvent that's used, and we tend to measure the volume of the solvent using a measuring cylinder.

Now, different situations will call for different volumes depending on whether or not you're doing an investigative reaction on a small scale or an industrial reaction on a large scale.

So it's really important that we understand that there can be different units for volume.

So 1000 millilitres is the same as 1000 centimetres cubed, which is the same as one decimeter cubed, and that is equal to one litre.

Now, those smaller units of millilitres and centimetres cubed tend to be units that we use on an investigative scale, things that we would be using in a science laboratory, whereas the other units are larger, those decimeter cubed and litres, so what we might use on an industrial scale.

So to swap between them, so an investigation that we might do in the laboratory, we might want to upscale to an industrial level.

So to change from cubic centimetres to cubic decimeters, I would simply divide that value by 1000.

Likewise, if I wanted to scale downwards from an industrial level to an investigative level, to change from decimeter cubed to centimetres cubed, I would multiply by 1000.

Now remember, a measuring cylinder is usually used to measure volume, and these crucially measure volume in centimetres cubed.

But when we're quoting the concentration of a solution, the standard unit for volume is quoted in decimeters cubed.

So we'll need to be doing some unit conversion.

Now, there's a quick and easy way that we can do this.

If we remember the rhyme C to the D, 1, 2, 3, you can easily convert between centimetres cubed and decimeters cubed.

So let's look at an example.

I have here 500.

0 centimetres cubed.

I'm going to change my C to a D simply by adding that line and then I'm going to move that decimal place three places to the left, 1, 2, 3, which helps me with that conversion then that 500 centimetres cubed is equal to 0.

500 decimeters cubed.

What I have done is divided by 1000 without actually having to use a calculator, changing the C to a D and moving that decimal place 1, 2, 3 to the left.

Let's stop here for a quick check.

What is the volume in decimeters cubed of 15.

2 centimetres cubed? Well done if you said C.

Now you could have done that by using a calculator and simply dividing 15.

2 by 1000, or you could have used our rhyme, C to the D, 1, 2, 3, and you get an answer of 0.

0152 decimeters cubed.

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

Great job, guys.

So we know now that a measuring cylinder can be used to measure out specific volume of solvent to create a solution, and how we can convert those units for describing concentration.

Let's move on to look at the solute in a bit more detail.

The solute's mass is usually measured using a balance, and that mass of a solute can be measured in lots of different units.

For instance, it could be in milligrammes or it could be even measured in kilogrammes.

However, the standard unit for mass of a solute is grammes.

So you may need to be doing a little bit of conversion of units for that standard unit of grammes here.

And what you'll notice, as we move from the smaller units of milligrammes up to grammes and on to kilogrammes, we're dividing by 1000.

And when you move from the larger units down through grammes and over to milligrammes on the left, we are multiplying by 1000.

So if I had one gramme of mass, that is equal to 1000 milligrammes, or 0.

001 kilogrammes mass.

Now, because a solution is composed of a solute that's been dissolved in a solvent, its concentration needs to be quoted in terms of the units for both.

Now, the units for concentration show a ratio of mass to volume.

So the standard units for concentration are going to be using the standard units for mass and volume, and those are grammes and decimeters cubed.

So the standard unit for concentration is grammes per decimeter cubed.

So that little slash line can be read as per.

We could also write this as grammes dm to the minus three, showing the grammes per decimeter cubed.

Let's stop here for another quick check.

Which of the following could be considered units for concentration? Well done if you said A, C, or D.

Now, C is the standard unit for concentration, but A and D could also be correct answers because they're showing that ratio of a unit for mass per unit for volume.

So very well done if you managed to choose C, but extremely well done if you also chose A or D.

Great job, guys.

You're doing fantastic work.

Keep it up.

So the concentration of a solution can be calculated using this equation of concentration is equal to mass divided by volume, and the equation can be rewritten slightly differently, looking like this.

So if you are in doubt and are struggling to remember this equation for concentration, let the units guide your calculation.

If we can remember that the units for concentration are grammes per decimeter cubed, we know that we're gonna take mass first, then divide it by the volume, which is in decimeters cubed.

Let's go through an example then.

I want to know what the concentration is in grammes per decimeter cubed of solution composed of 15 grammes sodium chloride dissolved in 0.

35 decimeters cubed of water, and to give that final answer to two significant figures.

So I'm going to use that equation of concentration equals mass divided by volume.

Now, because I have this concentration in grammes per decimeter cubed, I need to make sure that my values for mass and volume are in the correct units.

So you'll notice when I find those values within the question, I'm not just circling the number here, I'm circling the units as well.

So I have mass in grammes and my volume in decimeters cubed.

I don't need to do any unit conversion here.

I simply pop those numbers into my equation and I get a value of 42.

857.

However, I've been asked to quote my answer to two significant figures.

So my final answer is 43 grammes per decimeter cubed.

What I'd like you to do now then is to calculate the concentration in grammes per decimeter cubed of a solution that's composed of 5.

2 grammes sugar dissolved in 1.

6 decimeters cubed of water.

And again, to give your answer to two significant figures.

So what I'm gonna suggest you do is to pause the video while you do your working out and use what I have done as a guide to help you.

Okay, now, if you've carried out your calculations correctly, you should have had a final answer to two significant figures of 3.

3 grammes per decimeter cubed.

Very well done if you managed to get that, guys.

Great job.

Now, like most mathematical relationships, this equation for calculating the concentration of a solution can be rearranged so that we can calculate an unknown volume for the solvent or the mass of solute.

So our concentration equals mass divided by volume could be rearranged to show volume is equal to mass divided by concentration, or the mass of solute is equal to concentration times volume.

Now, the values for the solute and/or the solvent may need to be converted to the appropriate units before we actually calculate that solution's concentration.

So don't forget that volume needs to be in decimeters cubed and that the mass must be in that standard unit of grammes.

Now, when we're being asked to calculate an unknown value, it can be quite useful to employ a strategy.

So what I'm gonna suggest is that you first of all need to choose the appropriate equation for what you're being asked to calculate.

Are you being asked to calculate concentration, mass, or volume? Then you need to identify the values in your question and ensure that they are in the correct units for what you need, and that may need some converting.

Now remember, mass is usually in grammes, volume is usually in decimeters cubed, and your concentration tends to be in grammes per decimeter cubed.

Finally, once you have identified those values and ensured they're in the correct units you need, you will simply put those values into your chosen equation and solve for your unknown.

Okay, let's go through an example.

I'd like to know what mass of solute needs to dissolve in 0.

25 decimeters cubed of water to produce a solution that has a concentration of 2.

3 grammes per decimeter cubed.

And I want to have that final answer to two significant figures.

So I've been asked to calculate the mass of solute that is needed, and I've been provided with values that have units of decimeters cubed and grammes per decimeter cubed.

So I'm going to use this relationship of mass equals concentration times volume, and I've put the units that I need underneath each of these different variables so that I know which value to put where.

So I'm going to put 2.

3 grammes per decimeter cubed and I'm going to then multiply that by 0.

25 decimeters cubed, which is my volume, and that gives me a mass answer of 0.

575.

However, I need to give my answer to two significant figures.

So my final answer is 0.

58 grammes of solute is dissolved in 0.

25 decimeters cubed of water to give me the desired concentration of 2.

3 grammes per decimeter cubed.

What I'd like you to do now then is to calculate what volume would be needed to dissolve 1.

78 grammes of potassium sulphate to create a solution with a concentration of 1.

10 grammes per decimeter cubed, and to give your answer to two significant figures.

So you may wish to pause your video here so you can do your working out.

Perhaps use my example as a guide.

And then come back when you're ready to check your work.

Okay, let's see how you got on.

Now, if you've done your calculations correctly, you should have had a final answer of 1.

6 decimeters cubed to two significant figures.

So very well done if you managed to get that.

If not, you may wish to pause the video here so you can go back through your working out to find out where you may have gone wrong so we can try to avoid those errors in the future.

But well done for having a go at this, guys.

Great job.

Let's try another one.

What is the concentration in grammes per decimeter cubed of a solution that's composed of 2.

1 grammes of copper sulphate that's been dissolved in 125 centimetres cubed of water? And to give that final answer to two significant figures.

So the first thing I see is that I need to find the concentration, and that it's going to be in grammes per decimeter cubed.

So I'm going to use that mathematical relationship of concentration equals mass divided by volume.

And I've highlighted then the values that have been given in my question as well as those units.

And what I notice is that the units per volume are in centimetres cubed, but I need them in decimeters cubed.

So the first thing I'm going to do is to convert my volume to the correct units by dividing by 1000.

And that gives me a new volume of 0.

125 decimeters cubed.

I use that value along with 2.

1 grammes and put them into my relationship.

So 2.

1 divided by 0.

125 gives me a concentration of 16.

8.

However, my answer has to be to two significant figures.

So the final answer is 17 grammes per decimeter cubed.

What I'd like you to do now is to calculate for me the concentration in grammes per decimeter cubed of a solution that's composed of 0.

014 kilogrammes of sugar dissolved in 2.

7 decimeters cubed of water.

And please do give your answer to two significant figures.

Now, you may wish to pause the video so you can do your working out.

Perhaps compare your answers and your strategy with the people nearest you and then come back when you're ready to check your answers.

Okay, let's see how you got on.

So if you've done your calculations correctly, you should have a final answer to two significant figures of 5.

2 grammes per decimeter cubed.

Now crucially, you will have needed to convert kilogrammes into grammes.

Now it's worth remembering that calculations like this where you need to convert a unit, it's usually worth one mark.

So even if you forget to convert that unit, but you still show all your working out, you might achieve two out of three marks on this calculation rather than losing all of the marks.

So it's really important that you're showing all of your working out.

So even if you forget to convert, you still get the marks for processing those values correctly.

So well done for having a go on this one, guys.

Great job.

Keep it up.

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

In this first part, I'd like you to calculate the concentration in grammes per decimeter cubed of the following solutions.

So you may wish to pause the video here, make sure that you're showing all your working out, and then come back when you're ready to check your answers.

Okay, let's see how you got on.

So for these questions, you needed to use that mathematical relationship of concentration is equal to mass divided by volume.

So for letter A, you should have had a final answer of 28 grammes per decimeter cubed.

For B, you may have realised that you needed to convert the volume from centimetres cubed to decimeters cubed.

And once you've done that, your final answer then should be 1.

4 grammes per decimeter cubed again to two significant figures.

Very well done if you managed to get that correct.

Great job, guys.

For this next part, I'd like you to calculate the mass of solute required to create the following solutions and to give your answers to two significant figures.

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

Now, for these questions, you needed to use that mathematical relationship of mass is equal to concentration times volume.

So for letter A, you should have had an answer of 0.

39 grammes.

And for B, you may have seen that you needed to convert again the volume to decimeters cubed, and that gives you a final answer of 13 grammes to two significant figures.

So very well done for those.

Now that we know a little bit more about concentration and calculations, let's try and relate it a little bit more to solutions that we come across in our everyday life.

So for this next question, I'd like you to consider a mug of instant coffee and that those usually contain about five grammes of coffee grounds dissolved in 50 centimetres cubed of hot water.

So what's the concentration of coffee in grammes per centimetres cubed and in grammes per decimeter cubed? So pause the video, and come back when you're ready to check your answers.

Let's see how you got on.

For this question, you needed to use that mathematical relationship of concentration equals mass divided by volume.

So for letter A, you see that the values you were given for mass and volume were in the units you needed for concentration of grammes per centimetre cubed.

So your final answer should be 0.

1 grammes per centimetre cubed.

For B, we needed to convert the volume from centimetres cubed to decimeters cubed before completing our calculation.

And that final answer then would give us for B 100 grammes per decimeter cubed.

Well done if you managed to get this correct.

Now, for the final question of today's lesson, I'd like us to consider another common solution, children's medicines.

Now, some children's medicines contain paracetamol, which is a painkiller, and the concentration of paracetamol in a solution of children's medicine is 24 grammes per decimeter cubed.

Now, a single dose should contain 120 milligrammes of paracetamol.

I'd like you to calculate what volume of medicine, in centimetres cubed, will contain the required mass of paracetamol.

Pause the video here, show your workings out, perhaps work with another person to compare your strategies, and then come back when you're ready to check your answer.

Okay, let's see how you got on, because this was quite a tricky question.

You were asked to find the volume of medicine.

So we're using that mathematical relationship of volume equals mass divided by concentration.

We then have the units for our concentration in grammes per decimeter cubed and our mass in milligrammes.

Now we need to convert that mass then to the standard unit of grammes, and we do that by dividing by 1000.

So our mass now is 0.

120 grammes.

And if we divide that by our concentration of 24, we get a volume of 0.

005 decimeters cubed.

However, you were asked to give your final answer in centimetres cubed.

So we need to multiply our volume by 1000 to change from decimeters cubed to centimetres cubed, and your final answer should be 5 centimetres cubed.

So incredibly well done if you managed to get that correct, guys.

I'm very impressed.

Wow, we've gone through a lot in today's lesson.

So let's just summarise what we've learned.

Well, we learned that the concentration of an aqueous solution is the mass of solute that's been dissolved in a specific volume of solvent, usually distilled water, and that its units are grammes per decimeter cubed.

We've also learned that if we dissolve more solute in that volume of solvent, we create a more concentrated solution.

But we've also learned that a solution can be diluted or made less concentrated by increasing the volume of that solvent by adding distilled water.

And we use distilled water to ensure that there are no other substances present in the solution that we're forming.

We've also learned that the volume of solvent and the mass of solute can be measured in various units, but that those units can be easily converted so that we can calculate concentration in that standard unit of grammes per decimeter cubed.

I hope you've had a good time learning with me today.

I've had a great time learning with you, and I hope to see you again soon.

Bye for now.