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Welcome to today's lesson on relative formula mass.

The mass number in this lesson is taken to one decimal place, and 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 the 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 all about relative formula mass, and then we're going to look at how to calculate percentage by mass in a compound.

Our first learning cycle is relative formula mass.

So let's get learning.

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

Let's have a look at this example.

So here's neon and neon's got a relative atomic mass of 20.

All we're going to do is we're going to look for the larger of the two numbers.

In this periodic table it's displayed at the bottom.

The relative formula mass is just adding all of those relative atomic masses up.

So we're going to look at the relative formula mass, sometimes shortened to RFM, and it's also known as M subscript R.

For a compound is a sum of the relative atomic masses, so that's AR of the atoms shown in the formula.

Let's have a look at an example.

So what we've got here is we've got sodium, oxygen, and hydrogen atoms, and we're looking for the larger of the two numbers for each of these elements.

So for example, sodium hydroxide is a compound and it's got a relative formula mass of 23, so you can see it's sodium.

We're going to look at that sodium atom and we're going to see that larger number is 23.

We're going to add that to oxygen, which is 16.

0, and we're going to add that to the AR of hydrogen, which is 1.

So we're gonna add this up, and the sum of that is 40.

So then we say sodium hydroxide has got a relative formula mass of 40.

A quick check for understanding.

Which image shows a relative atomic mass AR for fluorine? Pause the video mow for some thinking time.

Press play and I'll tell you the answer.

Well done If you said that it's A, 19.

That's the relative atomic mass of fluorine.

It's the larger of the two numbers that you're looking for.

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

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

This sounds a little bit complicated, but let's have a look at a couple of examples to show you how straightforward it is.

So here's some elements from the periodic table and their information there, and we're going to calculate the MR for water, so this is H2O.

You'll see that little subscript two.

That means that we've got two hydrogen atoms and one oxygen atom.

So all we've got to do is do two, multiply that by 1.

0, so that's the AR for hydrogen.

Add that to the AR for oxygen equals 18.

Got another example here for propane.

We've got C3H8, so all we've got to do now is we've got three carbons because that little subscript three is next to the carbon, and we've got eight hydrogens.

So all we've got to do is three multiplied by 12.

0 because we've got that information from the periodic table.

Add that onto eight multiplied by 1.

0 'cause that's the AR for hydrogen equals 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.

So you've got three choices here, A, B, or C.

Pause the video and then press play when you've chosen what answer you're going to do.

Well done if you said B, 12.

So that's representing the carbons AR, add that two, you've got two oxygen.

So it's two multiplied by the A alpha.

Oxygen gives 16.

Use your calculator and you should get 44.

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

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.

So I want you to pause the video now and then press play when you've decided if it's true or false.

Well done if you said that that was indeed false.

Now I want you to justify your answer so you've got two choices to choose from here.

There is one carbon atom and four hydrogen atoms, so the relative formula mass is equal to four times 1.

0 plus 12 equals 16.

0, or there is one carbon atom and four hydrogen atoms. So the relative formula mass equals four multiplied by 1.

0 plus six equals 10.

Pause the video for some thinking time, press play and we'll go through the answer.

Well done if you said it was A.

There is one carbon atom and four hydrogen atoms. So the relative formula mass equals four multiplied by 1.

0 'cause there's four hydrogen atoms, and add that to 12, and that's the AR for carbon equals 16.

So remember the AR is the larger of the two numbers, so well done if you got that one correct.

The MR can be calculated from a displayed formula.

So let's have a look at some displayed formulas.

Here's ethene, ethene is made up of carbon atoms so we're going to count up the carbon atoms first of all.

So I can see I've got two there.

We're going to count up the hydrogen atoms. We can see that I've got four.

So the formula must be C2H4.

If I want to calculate the MR from that, all I've got to do is account for the number of atoms of each of those elements.

So I've got two carbons, multiply by two, multiply by 12.

0, add that to four, multiply by 1.

0, and that equals to 28.

We'll have another look at this one here.

So this molecule is called chloroethane.

We're going to do exactly the same, we're going to count up the carbon atoms. So we just need to count up each type of atoms. Count up the hydrogen atoms so we've got two carbon atoms, we've got five hydrogen atoms, and then we've just got one chlorine atom.

Then we're going to make that into a formula, so C2 H5 Cl, so that should be a lowercase.

And those numbers should make sure that they are in subscript to that smaller and lower than the letter representing the element, and then we can calculate the relative formula mass.

So we've got two carbons, so that's two times 12.

0, five hydrogens, five times 1.

0, and just one chlorine, so 35.

Total for that is 64.

I've got three images here of displayed formula, which image or images has got the formula C3 H6? 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 got that one correct.

In a balanced chemical equation, the mass is always conserved, and what this means is the sum of the MR of reactants equals the sum of the MR of products.

So this Greek letter here means sum of, just means add them all up.

So here we've got calcium carbonate, which breaks down when heated up into calcium oxide and carbon dioxide.

Here's the three elements that are involved in this reaction so you don't need to look them up on a periodic table, and we're gonna calculate first of all the MR for the reactant.

So the Mr for calcium carbonate is 40.

1, add that to 12 for the carbon, add that to three, multiplied by 16.

0, and you should get 100.

Then if we have a look at the products now, so here the products calcium oxide and carbon dioxide, all we've got to do is calculate the MR for these products.

So that's calcium oxide, so that's 40.

1, add 16.

0 gives 56.

1, and then we do the same for the carbon dioxide.

So we've got 12 for carbon, two oxygen, so we're gonna multiply that by two, and that equals to 44.

Now all we've got to do is add up for the products, so we're going to sum those.

So the sum of the MR for the products is equal to 100.

So you can see we've got a 100.

1 for the reactants, and a 100.

1 for the products.

It's the same.

We say that this is conserved.

If the MR of the reactants is 120.

0, what would the MR of the products be? Would it be higher? So over 120.

0 the same, so exactly 120.

0, or would it be lower less than 120.

0? Pause the video now, choose your answer, and press play and I'll tell you if you're right or not.

Well done if you said it would be B.

Remember we said that the relative formula mass is conserved for a balanced equation, so it's going to be the same.

Now we're going to look at another example here of calculating the MR of the reactants and the MR of the products.

So first of all, let's have a look here.

We've got methane, so we're gonna look at CH4, calculate that we should get 16.

0 for the reactant, and then we've got another reactant.

This time we've got two lots of oxygen, so we've got to account for that in our calculations.

So we've got two times 16, and then we're going to multiply that by number two.

Again, because a big number two in front of them in the equation to balance it up, that multiplies all those atoms that that's in front of by two.

So you should have got 64 for that.

16 add 64 should equal 80.

0, so well done if you got that.

What I want you to do now is I want you to calculate the sum of the MR for the products.

So pause the video while you do this, then press play when you've got your answer.

Well done for completing that check for understanding.

So here's the MR, so the relative formula mass for carbon dioxide equals 44.

Now here I hope you didn't get caught out by that big number two in front of the water.

So you've got two lots of water in title, so you work out the MR for water and then you multiply that by two so you should have got 36 there.

And then if we add those together, so we sum up the MR for the products, we should have 44.

0, add 36.

0, equals 80.

So well done if you've got that correct.

You can see that they've got the same values.

So it is true that the sum of the MR of the reactants equals the sum of the MR of products.

Well done if you've got that correct.

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 task A.

Using the periodic table to calculate the relative formula mass for the compounds.

So first of all, we've got hydrogen chloride, that's 36.

5, got potassium hydroxide 56.

1, sulphur dioxide 64.

Remember that little number two multiplies just the oxygens.

We've got nitrogen oxide there, so that's 44.

Again, that number two only multiplies the nitrogens, and then we've got this compound here, vanadium oxide, and you can see there that we've got two vanadium and we've got five oxygens.

So we should have a total of 181.

8 for the relative formula mass.

Let's go through the second part of the task now.

So we've got hydrogen peroxide, there you go 34.

Copper sulphate, 159.

6 H2 SO4, 98.

We've got silver nitrate, 169.

Remember the little subscript only multiplies just what it's next to, so for that one we've just got three oxygens.

And then we've got ion oxide, two ions, three oxygens gives you a total of 159.

6 for the relative formula mass effect.

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

You've done really well.

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.

Here's the answers.

So ethanol C2H6O, and here's the relative formula mass.

Two times 12, six times 1.

0, plus 16 for the oxygen gives 46.

Tetrafluoroethene, let's look at the formula for that.

C2F4, and here's the MR, exactly 100.

Ethanoic acid, here's the formula for this C2H4O2, and the relative formula mass should have been 60.

I'll have a look at propane C3H8, so that's 44.

0 exactly.

And phosphoramide looks a little bit more complicated.

Hopefully you didn't get caught out by that.

So you've got 1P1O, six hydrogens and three nitrogens so you should have got 95.

0 for that.

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

You're doing really well with relative formula mass now.

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 a 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.

For this equation, you should have shown all of your calculations.

So the first thing we're gonna do is we're gonna calculate the sum of the MR for the reactants.

So we look at sodium chloride's MR is 58.

5, and then we look at silver nitrate, which is 169.

Add those together, which is 228.

4, and then all we've got to do is do the exact same.

Calculate the relative formula mass of the products.

All you've got to do now is calculate the MR for the products.

So you should get 85.

0 for sodium nitrate, 143.

4, sum of those together so add them up.

Then you should see that you get the same values of 228.

4 for the sum of the MR of the reactants and the sum of the MR for the products.

So you can see that it's got the same values.

So that equation is correct.

Well done for completing that.

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

Now we're going to move on to learn how to calculate the 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 an element in compound and the MR, so the relative formula mass of a compound.

Here's the equation that you're going to use.

Percentage of element in compound, equals massive element in compound, divided by the relative formula mass.

Multiply that answer by 100.

Give you the percentage of element in a compound.

So here's our example, find the percentage of carbon and carbon dioxide.

So here's the mass of carbon and carbon dioxide, it's 12, and the MR so the relative formula mass is 44.

So we're just using the skills that we've developed from the first learning cycle to be able to do that, and the percentage of carbon and carbon dioxide, we're going to take 12 and divide that by 44, multiply that by 100, and that should give us 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.

We're now going to practise using the equation.

So here's the equation that we've seen before.

I'm going to give you an example, and then you're going to have a go at one yourself.

So calculate the percentage of hydrogen in methane.

So the mass of hydrogen, so you can see that we've got four hydrogens.

So we need to account for the fact that we've got four hydrogen atoms, so four times 1.

0 equals four.

The MR so the relative formula mass for methane is 12 plus four times 1.

0 equals 16.

0, and the percentage of hydrogen in methane all we're gonna do is we're gonna plug those numbers into that equation above.

So 4.

0 divided by 16.

0 multiplied by 100 equals 25%.

Now your turn.

Calculate the percentage of magnesium in magnesium chloride.

Pause the video now.

Carry out your calculation, and press play and we'll go through the answers together.

Well done for completing this.

Here's the mass of magnesium, it's 24.

So the MR for magnesium chloride, so the relative formula mass equals 95.

All you've got to do then is look at the percentage of magnesium in magnesium chloride.

24.

3 divided by 95.

3 multiplied by 100 equals 25.

5%, and that's to one decimal place.

So well done if you got that correct.

"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, and the formula for silver sulphide is Ag2S.

Then 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 working so hard.

Let's take a look at the answers.

So here's Sam's problem that she had.

So the massive silver in silver sulphide, we need to take account for the fact that these two atoms of silver.

So we multiply 107.

9 by two to give us 215.

Then we're going to work out the relative formula mass for silver sulphide, which is 247.

9, then all we've got to do is put the numbers into that equation.

So 215.

8 divided by 247.

9, multiply those up by a 100 should give you 87.

1% silver in silver sulphide.

Now we're going to have a look and see the percentage of silver in silver nitrate.

So the same process.

This time we've only got one atom of silver, so it's 107.

9, and then the relative formula mass for silver, the MR is 169.

Put those into our equation, so the percentage of silver and silver sulphide.

107.

9 divided by 169.

9, multiply that by 100 should give us 63.

5% silver in silver nitrate.

So was Sam correct? Yes, Sam was correct.

They said, "I was correct.

There is more silver in silver sulphide than silver in silver nitrate." So well done if you predicted that Sam was correct.

Now we're going to take a look at Andeep's problem.

The mass of sulphur in H2 SO4 is 32.

1, and that is the relative formula mass for sulfuric acid is 98.

All we've got to do is put those numbers into the equation again.

So 32.

1 divided by 98.

1 multiplied by 100 should give us 32.

7% of sulphur in sulfuric acid.

So unfortunately, Andeep said, "I was wrong.

Less than 50% of sulfuric acid is sulphur." So well done if you said that Andeep was incorrect.

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.

We're going to calculate the percentage by mass now of magnesium in magnesium oxide.

So this one's quite straightforward.

So 24.

3 is the mass for magnesium.

Here's the relative formula mass for magnesium oxide, 40.

3, and then all you've got to do is put those into this equation and times it by 100 should give you 60.

3% of magnesium in magnesium oxide.

So well done if you've got that one.

And then we do the same method here for potassium and potassium hydroxide here.

So there's just one potassium atom there, so that's 39.

Here's the MR for potassium hydroxide, 56.

Then all we've got to do is put those into the equation, multiply it out, and you can see that you get 69.

7%.

So well done if you've got those correct.

Now we're going to calculate the percentage of zinc in zinc chloride.

So quite straightforward again.

Got the massive zinc in zinc chloride, so that's 65.

Then we're gonna look at the relative formula mass for zinc chloride, which is 136.

All you've got to do then is put those numbers into that equation, multiply it out, and you should get 47.

9%.

Next we're going to calculate the percentage of hydrogen and hydrogen peroxide.

This time we need to account for the fact that we've got two hydrogen atoms when calculating the mass of hydrogen.

So here's the mass of hydrogen, two times 1.

0 equals 2.

Then we're going to calculate the relative formula mass or the MR for hydrogen peroxide comes out at 34%.

Sorry, at 34.

And then we've got to calculate the percentage of hydrogen and hydrogen peroxide.

Just put those numbers in, multiply it by 100, and it should equal 5.

9%.

Well done if you've got that.

Well done, you're doing really well for calculating all these percentage by masses.

So now we're going to look at manganese in potassium manganate.

So here's the mass of manganese, which is 54.

There's a relative formula mass at 158.

So all we've got to do is divide one by the other, so 54.

9 divided by 158.

0, multiply that by 100 should give you 34.

7% manganese in potassium manganate.

Well done if you got that one.

You're doing really well so far.

We're going to look at the amount of oxygen in calcium carbonate.

So remember to account for the fact that there's three atoms of oxygen in calcium carbonate.

So we're going to do three multiplied by 16.

0, which is 48.

0, and then there's the relative formula mass of 100.

Put those into our equation.

So the percentage of oxygen in calcium carbonate 48.

0, divided by a 100.

1, multiplied by 100 should give you 48.

0%.

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

We are now calculating the percentage by mass of carbon in glucose.

So quite simple here.

You've got to just account for the fact that you've got six carbon atoms. So six multiplied by 12.

0 gives 72.

Then once you've calculated the relative formula mass of 180.

0, you can just put those into the equation.

72.

0 divided by 180.

0 multiplied by 100 should give you 40.

0% of carbon in glucose.

Well done if you've got that one correct.

Now onto the last one, carbon in paracetamol.

So all we've got to do is account for the fact that you've got eight carbons there.

So eight multiplied by 12.

0 gives 96.

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

Although the formula's a little bit longer than the others, it's still calculated exactly the same way.

So you should get 151.

0, put those numbers into our equation, and you should come out with 63.

6% of carbon in paracetamol.

Well done if you've got all of those correct.

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 massive element in compound divided by MR. So that's a relative formula mass multiplied by 100.

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.

I've finished the video