Lesson video

Welcome to today's lesson on relative formula mass.

It's part of the unit, atomic structure and the periodic table.

My name's Mrs. Mitam-Smithson.

In this lesson we're going to calculate the relative formula mass of substances using data from a periodic table.

You'll need both a calculator and a periodic table for this lesson.

Don't worry if you get a little bit confused with maths.

The maths is actually quite simple in this lesson and we'll work through plenty of examples step by step.

And by the end of this lesson I'm sure you're going to be much more confident with these calculations.

Today's keywords are relative atomic mass, relative formula mass.

On this slide you'll see some sentences.

Pause the video if you wish to read them and then press play when you want to get going with the lesson.

Today's lesson consists of two learning cycles.

Firstly, we're going to learn how to calculate relative formula mass.

Then we're going to look at calculating percentages by mass in a compound.

First learning cycle is relative formula mass.

Let's get learning.

The relative atomic masses, A subscript R of atoms are shown on the periodic table.

Here's neon.

So neon has got a relative atomic mass of 20.

You can see that that's just the larger of the two numbers.

The relative formula mass, sometimes known as RFM for short is also known as M subscript R for a compound, is the sum of the relative atomic masses.

So that's Ar of the atoms shown in the formula.

That sounds really complicated, but let's break it down.

So here's three elements, sodium, oxygen and hydrogen.

And they're contained in sodium hydroxide.

So for example, when we're calculating the relative formula mass of sodium hydroxide, all we're going to do is take the relative atomic mass of sodium, add it to the relative atomic mass of oxygen and add that to the relative atomic mass of hydrogen.

So we'll see that sodium's relative atomic mass is 23.

Add that to oxygen's, which is 16.

Add that to hydrogen's, which is one.

I'm gonna add that up and that's going to equal 40.

So sodium hydroxide has a relative formula mass of 40.

Which image shows the relative atomic mass, Ar for fluorine? So pause the video, have a little think and then press play and I'll give you the answer.

Well done if you said 19.

It's the larger of the two numbers.

So fluorine has a relative atomic mass of 19.

Well done if you've got that one right.

If there's more than one atom of an element in a substance, then you need to account for this when calculating the relative formula mass of a compound.

Sounds a little bit complicated, but let's look at a couple of examples.

So here we've got some elements, hydrogen, carbon, and oxygen.

These are just taken directly from the periodic table.

And what we want to do is calculate the Mr of water.

So that's H2O.

You can see that little two, that subscript two is next to the hydrogen.

So that means that we've got two atoms of hydrogen.

So we need to multiply that by its relative atomic mass.

So two multiplied by one, 'cause that's relative atomic mass, and I'm gonna add that to 16, gives us a relative formula mass of 18.

Let's have a look at another one.

This was propane.

So here's the Mr. So we're looking for the relative formula mass of propane.

So we've got C3, so we've got three carbons and H8 so we've got eight hydrogens.

All we've got to do is we're going to multiply 3 multiply it by 12, because that's the atomic mass of carbon, we're gonna add that to 8 'cause we've got 8 hydrogens, multiply it by 1 because that's the Ar for hydrogen.

We're going to then add those up together and we're gonna get a total of 44.

Now what I want you to do is tell me what is the correct method of calculating the Mr of CO2? So that's carbon dioxide.

I'm gonna give you three examples here, three choices.

So have a look at those.

So we've got 12, add 2 times 8 equals 28, and we've got 12 add 2 times 16 equals 44 or we've got 16, add 2 times 12 equals 40.

So pause the video, make your choice and then we'll press play and then I'll give you the answer.

Well done if you said it was B.

12 represents the carbon's Ar, so we're going to have 12 plus we've got two of them 'cause it's that little subscript two next to the oxygen and the oxygen's Ar is 16, so we've got 2 times 16 equals 44.

So well done if got that one correct.

A quick check for understanding now, true or false.

For the formula CH4, there are four carbon atoms and four hydrogen atoms. The relative formula mass or Mr for CH4 is 52.

Is that true or is that false? Pause the video now and press play and we'll give you the answer.

Well done if you said that that was false.

So what is the justification for that answer? So you've got two to choose from.

There is one carbon atom and four hydrogen atoms. So the relative formula mass is 4 times 1 plus 12 equals 16.

Or is it, there is one carbon atom and four hydrogen atoms so the relative formula mass is 4 multiplied by 1 plus 6 equals 10? Pause the video, make up your mind which one it is and then press play and I'll tell you the answer.

Well done if you said it was A.

We've got four hydrogens, so we're going to multiply that by 4 and then we're going to add that to 12 and that total is 16.

So well done if you got that correct.

The relative formula mass can be calculated from a displayed formula.

So we're going to have a look at some displayed formulas.

So here's one displayed formula, this is ethene.

All we're going to do now is we're going to count up those carbon atoms. So we're gonna count up the C's.

You can see that we've got two there.

We're gonna count up the hydrogen atoms. You can see we've got four there.

So the molecular formula must be C2H4.

Now remember those little numbers should be subscript.

So that means that they're smaller and lower than the capital letter that you've used to represent the element.

If you want to calculate the Mr for this, all we've got to do is we've got two carbon atoms, so that's 2 times 12, and we've got four hydrogen atoms, plus 4 times 1, that equals to 28.

Let's have another look at another example.

So this displayed formula just displayed three dimensionally.

First of all, this molecule's called chloroethane.

We've got to count up the carbon atoms. So you can see from that we're gonna count up the C's.

So that's two of them.

We're going to count up the hydrogen atoms, see one, two, three, four, five, five of them.

And we're gonna count up the chlorine atoms. So Cl, we've only got one of those.

So the formula is C2H5Cl.

So that L must be a lower case L.

And we don't write the number one.

So let's calculate the Mr for it.

So Mr equals 2 times by 12.

So that's the Ar for carbon plus 5 multiplied by 1, that's the Ar for hydrogen plus the Ar for chlorine, which is 35.

5 and the total for that, so the Mr equals 64.

5.

So the relative formula mass for chloroethane is 64.

5.

I've got three images here of displayed formula.

Which image or images has got the formula C3H6? Pause the video now and then press play when you've thought about the answer or answers, and I'll tell you if you're right or not.

Well done if you said A and B.

A and B are absolutely correct.

You can see that you've got three carbon atoms and you've got six hydrogen atoms. Well done if you've got that one correct.

In a balanced chemical equation the mass is always conserved.

What this means is that the sum of the relative formula mass of the reactants equals the sum of the relative formula mass of the products.

This symbol here, this Greek letter here means the sum of.

So we're going to look at an example here.

We've got calcium carbonate, we heat that up and that decomposes so it breaks down into calcium oxide and carbon dioxide.

These are the three elements involved.

So here's our information that we need to calculate the relative formula mass with.

So we're gonna have a look at the reactants or in this case we've got one reactant.

So the Mr for the reactant is equal to calcium carbonate, so that's equal to 40 plus 12 plus 3 multiplied by 16, 'cause we've got three oxygen atoms. The total for that is a hundred.

Now if we have a look at the products from that, if we first work out the Mr on the relative formula mass for calcium oxide, we can see that that equals 56.

And then we have a look at the other product, which is carbon dioxide and we calculate the relative formula mass for that.

That equals 44.

And we'll see that when we add 56, add 44 to the relative formula mass for the products in total it equals to a hundred.

So you can see there that both the reactants and the products, the mass has been conserved.

It means that they're the same.

If the Mr of the reactants is 120, what would the Mr of the products be? Would it be higher than 120? Would it be the same? So 120 exactly.

Or would it be lower? So less than 120.

Pause the video now and then press play when you've got an answer.

Well done if you said it would be the same.

Because the mass is always conserved if the Mr of the reactants is 120, then the Mr of the products must be 120 too.

Well done if you got that one right.

We're going to take another look at an example to show that the sum of the Mr of reactants is equal to the sum of the Mr of products.

So we're gonna have a look at this equation here.

She's methane add oxygen makes carbon dioxide and water.

So this is combustion reaction here.

So first of all, we're going to calculate the Mr for CH4 so for methane.

That's quite straightforward.

12 add 4 multiply by 1 is equal to 16.

Now this next part is a little bit trickier because we're going to do, we need to calculate the relative formula mass for 202.

So we've got O2, which is 2 multiplied by 16, and then this big number multiplies all the atoms by 2.

So we're going to multiply everything that we've got.

2 times 16 multiplied by 2 equal to 64.

Then all we've got to do is sum those.

So we're going to add those up.

So 16 add 64 is equal to 80.

What I'd like you to do is calculate the sum of the Mr of the products and then we're going to see if you can get the same answer that I've got.

So pause the video now, carry out that tiny little check for understanding and then press play when you've got your answer.

Well done for completing that.

Let's see how you got on.

So the Mr for CO2 is equal to 12 plus 2 times 16 equals 44.

Then you've got two lots of water.

So you've got H2O, multiply that by 2, 2 multiplied by 1 plus 16 and then multiply all of that by 2 should give you 36.

36 add 44 is equal to 80 and you should be able to see that you've got the same values for each.

So well done if you've managed to show that you've got the same values for each.

Here's task A now.

Using a periodic table, calculate the relative formula mass or Mr for the following compounds.

Pause the video while you do this and then press play and we'll go through the answers together.

Well done for completing that task.

Let's go through the answers now.

So for hydrogen chloride, you should have done one, for the hydrogen plus 35.

5 should give you a total of 36.

5.

Potassium hydroxide.

You should have a total of 56.

For the relative formula mass there, sulphur dioxide, again, you should have a relative formula mass of 64.

Nitrogen oxide, 44, vanadium oxide, 182.

So well done if you've got all of those right.

Now we're gonna go through the next few answers.

So we've got hydrogen peroxide is 34, copper sulphate 159.

5, H2SO4, now for this one, remember that oxygen is only multiplied by 4 and not the sulphur.

So well done if you've got 98 for that one.

Bit tricky.

Silver nitrate, we've got 108 for silver, 14 for the nitrogen, and then we've got 3 multiplied by the 16 because the little subscript 3 only multiplies the oxygen, so that's 170.

So well done if you've got that one right.

And finally we've got iron oxide.

This time we've got two irons, three oxygens, and a total of 160 for the relative formula mass of iron oxide.

So well done for completing all of those.

For part two of task A, what I want you to do is write the formula and calculate the relative formula mass for the compounds pictured.

So pause the video whilst you do that, press play and I'll tell you the answers.

Well done for completing that task.

So ethanol, we've got C2H6O and the relative formula mass for this is 46.

We've got tetrafluoroethene, which is C2F4.

So the Mr for this is we've got four lots of 19 and two lots of carbon should equal a hundred.

Ethanoic acid, C2H4O2.

So the Mr for this is 2 times 12, 4 times 1, 2 times 16 equals 60.

We've got propane.

We've got C3H8 and total Mr so the total relative formula mass of 44.

And the last one, phosphoramide.

Looks a really complicated formula but provided you just did exactly the same technique, you should have managed to get it correct.

So for this one you've got POH6N3.

And then you just calculate that one out and you got a total relative formula mass for 95.

So well done if you've got those.

Here's part three of task A.

For this equation I would like you to show me that the sum of the relative formula mass of the reactants equals the sum of the relative formula mass of the products.

I want you to show all of your calculations for this.

So pause the video now, complete this task and then press play when you've got your answer.

Well done for completing that task.

Let's have a look how you got on.

So what you should have done here is calculate the Mr for NaCl, 58.

5 and the Mr for silver nitrate, which is 170.

Add those together.

So that's the sum of the reactants equals 228.

5 and then all you've got to do now to prove that it's correct is do the same for the products.

So this time we've got sodium nitrate and we've got silver chloride.

So at 85 and 143.

5.

Add those together and you should see that the mass of the reactants is the same as the mass of the products.

So you can see that they've got the same values.

So well done if you've got that correct.

We've completed our first learning cycle about relative formula mass.

Now we're going to look at percentage by mass in a compound.

You can calculate the percentage by mass of an element in a compound if you know two things.

So the mass of that element in a compound and the Mr of a compound.

So here's an equation that shows you what you need to know.

So find the percentage of carbon in carbon dioxide.

So all we've got to do for this is the mass of carbon equals 12 and the Mr for carbon dioxide is 12 plus 2 times 16 because the Ar for oxygen is 16 equals 44.

So the percentage of carbon in carbon dioxide is 12 divided by 44, then multiply the answer by a hundred and that should give you 27.

3% to one decimal place.

So there is 27.

3% carbon in carbon dioxide.

Here's a quick check for understanding.

To calculate the percentage by mass of an element in a compound, you need to know, the element's atomic number only, the mass of an element in a compound only, only the relative formula mass of the compound, or both the mass of an element in a compound and the relative formula mass of that compound.

Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said D, both the mass of the element in the compound and the relative formula mass of the compound are the two things that you need to know to calculate the percentage by mass of an element in a compound.

I'm going to show you an example now and then you're going to have a go at one of these calculations.

So here's another example.

So calculate the percentage of hydrogen in methane.

So the first thing we need to do is calculate the mass of hydrogen.

So we need to account for how many atoms of hydrogen are in that compound.

So we're going to look at the formula and we can see that it's CH4.

So there's four hydrogens there.

So that's why we've got 4 multiplied by 1 equals 4.

So that's the mass of hydrogen.

Then we're going to look at the relative formula mass for methane.

So we just calculate that out as we've done in the first learning cycle.

And then we can look at the percentage of hydrogen in methane.

So all we've got to do there is do the mass of hydrogen, which is 4 divided by 16, which is the Mr for methane multiplied by a hundred and you can see that's 25.

0%.

Now what I'd like you to do is have a go at calculating the percentage of magnesium in magnesium chloride.

Pause the video now, carry out that calculation and then press play when you've got your answer.

Well done for completing that task.

Calculate the percentage of magnesium in magnesium chloride.

So if we look at the equation, we need the mass of the element in the compound.

So the mass of magnesium is 24 and then the Mr for magnesium chloride, so the relative formula mass is 24 plus 2 multiplied by 35.

5 equals 95.

And then all we've got to do is put those numbers into that equation to calculate the percentage of magnesium in magnesium chloride.

So there you go, 24 divided by 95, multiply the answer by a hundred should give you 25.

3%.

So well done if you got 25.

3%.

Are Sam and Andeep correct? Answers should be given to one decimal place.

Sam says, "I think there is a greater percentage of silver in silver sulphide," the formula for silver sulphide is Ag2S, "than the percentage of silver in silver nitrate." The formula for silver nitrate is AgNO3.

Andeep says, "I think that more than 50% of sulfuric acid," the formula for sulfuric acid is H2SO4, "is sulphur." What I want you to do is use this equation here to see if Sam and Andeep are correct.

Answers should be given to one decimal place.

Pause the video now, carry out the task and press play when you've got your answers.

Well done for completing this task.

Let's see how you got on.

So the mass of silver in silver sulphide, we need to take into account that the fact that there's two atoms of silver there.

So we're going to multiply the Ar of silver by 2 to give us 216.

Then we got to work out the relative formula mass.

So all we've got to do there is 2 multiplied by 108 plus 32 equals 248.

Then the percentage of silver in silver sulphide, we're going to just put those numbers into the equation.

So 216 divided by 248, multiply that by a hundred, should give you 87.

1% silver in silver sulphide.

Now we're going to calculate the mass of silver in silver nitrate.

So this time there's only one atom of silver, so that's just 108.

And then the relative formula mass for silver nitrate is 170 by that calculation there.

Put the numbers into that equation.

108 divided by 170 multiplied by a hundred should give you 63.

5% silver in silver nitrate.

So was Sam correct? Yes, she was correct.

There is more silver in silver sulphide than silver in silver nitrate.

So well done if you got that Sam was correct.

Now we're going to take a look and see if Andeep's correct.

So the mass of sulphur in H2SO4 is 32, and then this is the relative formula mass for sulfuric acid of 98.

So the percentage of sulphur in sulfuric acid, all you've got to do is take 32 divided by 98 multiplied by a hundred, gives 32.

7% of sulphur in sulfuric acid.

So, Andeep was wrong because he said that there was less than 50% of sulfuric acid is sulphur and he'd predicted that 50% of sulfuric acid was sulphur.

So Andeep was wrong.

Sorry, Andeep.

Here's part two of task B.

What I want you to do is use the equation to calculate the percentage by mass of magnesium in magnesium oxide, oxygen in potassium hydroxide, zinc in zinc chloride, hydrogen in hydrogen peroxide, manganese in potassium manganate, oxygen in calcium carbonate, carbon in glucose, carbon in paracetamol.

The formula for these compounds are by the side of the names of them.

So pause the video now, complete the task and press play and we'll go through the answers together.

Well done for completing that task.

So calculate the percentage of magnesium in magnesium oxide.

Quite straightforward this one.

Only one atom of magnesium.

So 24 divided by the Mr for magnesium oxide.

So 24 divide by 40, multiply that by a hundred equals 60%.

So there's 60% mass of magnesium in magnesium oxide.

Now we're going to have a look at the percentage of potassium in potassium hydroxide.

Just the same way really.

Calculate the mass of potassium, calculate the relative formula mass.

So that's 56.

Put those into this equation, 39 divided by 56 multiplied by a hundred should give you 69.

6%.

So well done if you've got those two correct.

Calculate the percentage of zinc in zinc chloride.

So again, there's only one atom of zinc, so it's just 65.

And then we calculate the relative formula mass to 136.

Put those numbers in, multiply it by a hundred, gives 47.

8% of zinc in zinc chloride.

Next, calculate the percentage of hydrogen in hydrogen peroxide.

Remember this time you're gonna have to account for the fact that there's two atoms of hydrogen there.

So we need to multiply the Ar for hydrogen by 2.

Relative formula mass is 34 for hydrogen peroxide.

Put those numbers in and you should get 5.

9% hydrogen in hydrogen peroxide.

So well done if you've got both of those correct.

Let's have a look at the next one.

Manganese in potassium manganate.

So 55 for the mass of manganese, Mn.

And then we've got to calculate the relative formula mass, put them into the equation and we should get 34.

8%.

Now onto F.

So F is the amount of oxygen in calcium carbonate.

This time if you have a look, there's three atoms of oxygen.

So we need to take that into account by multiplying the Ar by 3.

So that gives 48.

The Mr is calculated, the relative formula mass to a hundred.

So all you've got to do is 48 divided by a hundred equals 48%.

So well done if you got those two correct.

Now we're looking to calculate the carbon in glucose.

So the mass of carbon.

So we need to account for the fact that there's six atoms of carbon in this formula.

So we're going to do 6 multiplied by 12, gives 72.

And then the relative formula mass of glucose is 180.

So the percentage of carbon in glucose is 72 divided by 80, multiplied by a hundred equals 40%.

So well done if you got that correct.

Well done if you've got this far.

So now we've got the most complicated one, which is carbon in paracetamol, but we just need to do exactly the same steps.

So we're going to account for the fact that there's eight carbon atoms. So the mass of carbon in paracetamol is 96.

Then we're going to calculate the relative formula mass.

Should get 151.

Put those into the equation.

96 divided by 151 multiplied by a hundred gives 63.

6%.

So well done if you got that one correct, you've done really well.

Here's a summary of the lesson, relative formula mass.

The relative atomic mass, Ar is the mean mass of an atom compared to 1/12th of the mass of a carbon-12 atom.

The relative formula mass, RFM or Mr, is the sum of the relative atomic masses of all the atoms in a chemical formula.

The percentage by mass of an element in a compound can be calculated by using this formula.

Percentage of element in compound equals mass of element in compound divided by Mr, so that's the relative formula mass, multiplied by a hundred.

Well done for working really hard this lesson.

I hope you now feel much more confident in calculating relative formula masses and the percentage of mass of an element in a compound.

I hope to see you again soon.