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# Chemistry help! watch

1. So I thought in the mass/molar mass= moles equation, mass is in grams and molar mass is grams per mole. So why are they doing 297kg/146.1 ?

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2. because it is in proportion, whatever the answer in grams is for 297 g will be the answer in kg for 297 kg
3. (Original post by trou_noir)
because it is in proportion, whatever the answer in grams is for 297 g will be the answer in kg for 297 kg
I still don't quite understand. Do you mean to say 297000g?
Also, the molar mass is grams per mole so I don't get how you can do 297kg divided by 146.1grams per mole - the units are not the same.
4. (Original post by Kalabamboo)
I still don't quite understand. Do you mean to say 297000g? Also, the molar mass is grams per mole so I don't get how you can do 297kg divided by 146.1grams per mole - the units are not the same.
They’re not doing 297 kg / 141.6 g

They’re doing 297 g / 141.6 g

The answer would be multiplied by 1000 to get the result for 1000 times the amount they have calculated, but then to get the result in kg you would just have to divide it by 1000 again, so they didn’t bother
5. ok keep it in g until the end then.

297,000 g / (Mr) 146.1 = 2,033 moles of SF6 reacted (4 s.f.)

As stoichiometric ratio is 8:1, then (8 x 2,033) moles Li reacted

= 16,260 moles Li reacted (4 s.f.)

As moles = mass (g) / Mr then, moles x Mr = mass (g)

16,260 moles x (Mr) 6.9 = 112,200 g (4 s.f.), therefore;

Ans = 112 kg (3 s.f.)
6. (Original post by trou_noir)
They’re not doing 297 kg / 141.6 g

They’re doing 297 g / 141.6 g

The answer would be multiplied by 1000 to get the result for 1000 times the amount they have calculated, but then to get the result in kg you would just have to divide it by 1000 again, so they didn’t bother
Thanks! So here's what I've got so far:
297g/141.6g -->2moles
therefore 297g=2moles
(muliply result by 1000times) 297000g=2000moles
297kg=2000moles?
7. (Original post by 303Pharma)
ok keep it in g until the end then.

297,000 g / (Mr) 146.1 = 2,033 moles of SF6 reacted (4 s.f.)

As stoichiometric ratio is 8:1, then (8 x 2,033) moles Li reacted

= 16,260 moles Li reacted (4 s.f.)

As moles = mass (g) / Mr then, moles x Mr = mass (g)

16,260 x 6.9 = 112,200 g (4 s.f.), therfore;

Ans = 112 kg (3 s.f.)
Thanks! But this is how I worked it out as well The mark scheme way is much cleaner but I can't get my head around that way atm
8. (Original post by Kalabamboo)
Thanks! So here's what I've got so far:
297g/141.6g -->2moles
therefore 297g=2moles
(muliply result by 1000times) 297000g=2000moles
297kg=2000moles?
No - I just meant at the beginning. You still have to do the rest of the calculation like the other poster

You answer needs to be in kilograms but because you are using moles you are doing your working in grams. They have just not shown all their working in the mark scheme.
9. (Original post by trou_noir)
No - I just meant at the beginning. You still have to do the rest of the calculation like the other poster

You answer needs to be in kilograms but because you are using moles you are doing your working in grams. They have just not shown all their working in the mark scheme.
It just doesn't seem to make sense Could you please show me your working if possible?
Btw thanks for your help anyways
10. (Original post by Kalabamboo)
It just doesn't seem to make sense Could you please show me your working if possible?
Btw thanks for your help anyways
OK I’ll give it a try don’t worry, I used to hate methods like this too

1 kg = 1000g

So to work out how much lithium you need for 297 kg, you’re working out how much lithium you need for 297000 g of SF6.

But that’s a big number, so if you work out how much lithium you need for 297 g of SF6, you can then multiply it by 1000 at the end.

It just so happens that this question wants the answer in kg. So once you’ve found the answer at the end, you need to put it in kg

So you divide it by 1000 again

Instead of doing that, the mark scheme has just said, let’s assume that whatever the answer is for 297 (g) is also the answer for 297 kg

they can ignore the units in this case because the number that’s valid if you assume everything is in grams, is also valid if you assume everything is in kilograms.

If you converted into grams (ie multiplied by 1000) first and then back into kg at the end, you still did it right.

In fact I would argue yours is the more conscientious method as it’s easy to confuse units in general and forget to convert
11. (Original post by trou_noir)
OK I’ll give it a try don’t worry, I used to hate methods like this too

1 kg = 1000g

So to work out how much lithium you need for 297 kg, you’re working out how much lithium you need for 297000 g of SF6.

But that’s a big number, so if you work out how much lithium you need for 297 g of SF6, you can then multiply it by 1000 at the end.

It just so happens that this question wants the answer in kg. So once you’ve found the answer at the end, you need to put it in kg

So you divide it by 1000 again

Instead of doing that, the mark scheme has just said, let’s assume that whatever the answer is for 297 (g) is also the answer for 297 kg

they can ignore the units in this case because the number that’s valid if you assume everything is in grams, is also valid if you assume everything is in kilograms.

If you converted into grams (ie multiplied by 1000) first and then back into kg at the end, you still did it right.

In fact I would argue yours is the more conscientious method as it’s easy to confuse units in general and forget to convert
Ahhh thank you very much!!This makes sense now
So I worked out 112.2g of lithium for 297g of SF6
Then,as you kindly advised, multiplied by 1000 so 112200g of lithium for 297000g of SF6
then divide by 1000 to get it in in kg .. so 122.2kg of lithium for 297kg of SF6

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