Help please! Moles - science ones not garden ones

[nancy75]

My daughter is doing chemistry and trying to figure out moles.

Can anyone explain, in very small words what the initial mole value is please?

How do you find/know the initial mole value?

I have googled a lot & feel like I would have more luck learning chinese with my eyes shut than understanding this

Thank you 🙏

Does this help? www.bbc.co.uk/bitesize/guides/z84wfrd/revision/2

A mole is the number of atoms or molecules and is a constant number. You learn it.
Then you can calculate the number of moles of your substance by using formulae that you learn.
Mol of substance = mass in g / molecular mass
Etc
think. ‘Twas long ago

A mole is 6.02 × 1023 so a mole of elephants would be 6.02 × 1023 elephants.

If you have a mole of carbon, you would have 6.02× 1023 carbon atoms. This has a mass of 12g. We can use this to work out the mass of lots of different elements.

That should be x10 to the power of 23 (not multiply by 1023)!

Number of moles = mass of stuff /molecular mass of the compound.

You get the molecular mass from the formula and using a periodic table.

Avogradros constant tells you how many items you have in one mole.
So if you have 12g of Carbon (molecular weight 12), you have 6×10^23 carbon atoms.
If you gave 58g of salt (sodium chloride, NaCl) you also have 6x10^23 salt molecules.

And you need moles because each substance has a different mass but (very bad example coming up) e.g. you need two hydrogen and one oxygen to make water. But if you take 10g hydrogen and 10g oxygen you won’t have enough oxygen to react with all the hydrogen because
1 mol hydrogen = 1 g;
1 mol oxygen = 16g

You need two hydrogen atoms I.e. 2g to react with 16g of oxygen to make sure all the atoms find a friend.
And it’s an arse to calculate with 6.02 x 10^23 each time so we use moles instead.

No of moles = mass (g) / molar (atomic) mass (g/mol)

So for example

Q:
Calculate the number of moles of carbon dioxide molecules in 22 g of CO2

Answer:
Atomic mass of C= 12
Atomic mass of O= 16
So... atomic mass of CO2 = 12+16+16= 44 g/mol

To find number of moles you divide the mass 22 (g) by 44 = 0.5moles

Argh sorry I thought you said working out how to find the amount of moles for some reason.

Yes in your case what she is looking for like a pp said is Avogadro's number

I'll also try to explain but I think bitesize may do it better. All atoms have an atomic number. That is the number of protons in the nucleus. They also have an atomic weight which measures the number of protons and neutrons in the nucleus. So helium would have an atomic number of 2 and an atomic weight of 4 because its nucleus contains 2 protons and 2 neutrons. A mole is the weight in gms that equals the atomic weight. So for helium it is 4gms, for say, carbon, 6 protons +6 neutrons, it would be 12 grams. To get the weight of one mole of a compound you have to add the atomic weights of all the atoms in that molecule. So sodium chloride NaCl would be 1 sodium, atomic weight 23 and one chlorine, atomic weight 35 giving the weight of one mole of NaCl as 23+35= 58gms Hope that makes sense

Thank you all so far, some of it we’re getting.
The thing confusing us is below

Converting from moles to mass (grams): Multiply your initial mole value by the molar mass of the compound as determined by the periodic table.
In the above what is the Initial mole value?

In that example 'initial mole value' seems to mean 'number of moles'.

What exam board is this? I teach chemistry and I've not heard it called that.

The initial mole value in that question would just be the number of moles you have.

@Frlrlrubert

In that example 'initial mole value' seems to mean 'number of moles'.

What exam board is this? I teach chemistry and I've not heard it called that.

She’s doing AQA but this is from chemistry for dummies (fitting for my level of knowledge!) We’ve been Googling it for ages & that phrase has been our sticking point, but if it’s not used I can tell her to ignore it

A mole is just a word for a number, like a pair of socks, or a dozen eggs.

The number is question is 6 x 10 to the power 23. So if you have a mole of anything, you have 6 x 10 to the 23 of it, be that thing chocolate bars, people, or carbon atoms.

So, we thing we’re getting it & then we’re not!

How do we do this?

48g of magnesium and 32g of oxygen react to form 80g of magnesium oxide. 2Mg + O2 2MgO
What mass of oxygen is needed to make 20g of MgO?
(4 marks)

This is not homework by the way, we’re not cheating just flummoxed

If 32g is required to make 80g MgO, then 20g is a quarter of 80... so you will need a quarter of the 32g ie 8g.

12g Mg and 8g O2 = 20g of MgO

Proportions
48:32 = 80
24:16 = 40
12:8 = 20

The above is what we thought, but the question would be worth 4 marks, so just dividing by 4 seemed too easy?
I can’t work out if we’re making it harder than it is!

You can make it more complicated by saying the molar mass of oxygen (atoms) is 16 (so 32g for O2), and magnesium is 24. So one mole of MgO weighs 40g. So 20g of MgO is half a mole. And half a mole of Oxygen is 8g. But it's not necessary and gets to the same answer

That question does seem too easy, normally they'd give you the equation and just ask you 'what mass of oxygen is needed to make 20 g of magnesium oxide?'. Without giving you the hint.

Then you'd have to work out that you have 0.5 moles of MgO (20/40), and need 0.25 moles of oxygen, so (32 x 0.25) 8 g.

Also, I'd have to double check for AQA, but shouldn't you be using relative atomic mass to 1 decimal place? So magnesium should be 24.3 (and MgO therefore 40.3).

@Frlrlrubert

Also, I'd have to double check for AQA, but shouldn't you be using relative atomic mass to 1 decimal place? So magnesium should be 24.3 (and MgO therefore 40.3).

This one was copied from an AQA paper, but yes I’m sure you’re right about the .1 bit.

I do really appreciate all the help, dd had got herself to the point of can’t do it having a breakdown & I haven’t done chemistry for 30 years!

Can’t beat a bit of chemistry on a Tuesday night!

Thank you all

This is why I love Mumsnet! Thanks OP, that has brought back some memories