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 ?
Attachment 730118730120
You are Here:
Home
> Forums
>< Study Help
>< Maths, science and technology academic help
>< Chemistry
|
Chemistry help! watch
-
Kalabamboo
- Follow
- 0 followers
- 13 badges
- Send a private message to Kalabamboo
- Thread Starter
Offline13ReputationRep:- Follow
- 1
- 09-03-2018 16:17
-
trou_noir
- Follow
- 0 followers
- 11 badges
- Send a private message to trou_noir
Offline11ReputationRep:- Follow
- 2
- 09-03-2018 16:21
because it is in proportion, whatever the answer in grams is for 297 g will be the answer in kg for 297 kg
-
Kalabamboo
- Follow
- 0 followers
- 13 badges
- Send a private message to Kalabamboo
- Thread Starter
Offline13ReputationRep:- Follow
- 3
- 09-03-2018 16:25
(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
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. -
trou_noir
- Follow
- 0 followers
- 11 badges
- Send a private message to trou_noir
Offline11ReputationRep:- Follow
- 4
- 09-03-2018 16:31
(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 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 botherLast edited by trou_noir; 09-03-2018 at 16:35. -
303Pharma
- Follow
- 8 followers
- 15 badges
- Send a private message to 303Pharma
Offline15ReputationRep:- Follow
- 5
- 09-03-2018 16:46
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.)Last edited by 303Pharma; 09-03-2018 at 16:52. Reason: typo's -
Kalabamboo
- Follow
- 0 followers
- 13 badges
- Send a private message to Kalabamboo
- Thread Starter
Offline13ReputationRep:- Follow
- 6
- 09-03-2018 16:49
(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
297g/141.6g -->2moles
therefore 297g=2moles
(muliply result by 1000times) 297000g=2000moles
297kg=2000moles? -
Kalabamboo
- Follow
- 0 followers
- 13 badges
- Send a private message to Kalabamboo
- Thread Starter
Offline13ReputationRep:- Follow
- 7
- 09-03-2018 16:52
(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.)The mark scheme way is much cleaner but I can't get my head around that way atm
-
trou_noir
- Follow
- 0 followers
- 11 badges
- Send a private message to trou_noir
Offline11ReputationRep:- Follow
- 8
- 09-03-2018 17:00
(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?
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. -
Kalabamboo
- Follow
- 0 followers
- 13 badges
- Send a private message to Kalabamboo
- Thread Starter
Offline13ReputationRep:- Follow
- 9
- 09-03-2018 17:09
(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.Could you please show me your working if possible?
Btw thanks for your help anyways -
trou_noir
- Follow
- 0 followers
- 11 badges
- Send a private message to trou_noir
Offline11ReputationRep:- Follow
- 10
- 09-03-2018 17:20
(Original post by Kalabamboo)
It just doesn't seem to make senseCould you please show me your working if possible?
Btw thanks for your help anywaysdon’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 -
Kalabamboo
- Follow
- 0 followers
- 13 badges
- Send a private message to Kalabamboo
- Thread Starter
Offline13ReputationRep:- Follow
- 11
- 09-03-2018 17:36
(Original post by trou_noir)
OK I’ll give it a trydon’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
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
- New gcse chemistry exams, help!!
- OCR Chemistry 2017 Practice Paper Question - HELP
- HELP A Level Chemistry keep failing
- stuck please help me organic chemistry
- Urgent chemistry please help
- Help me with the AS physics chemistry pdfs please
- Need some help with chemistry please.
- chemistry textbook for edexcel help
- Chemistry AS Level edexcel solubility help needed.
- CHEMISTRY aqa help -MCQ
-
Chemistry with Forensic Investigation
University of Surrey
-
University of Huddersfield
-
Manchester Metropolitan University
-
Chemistry, the Atmosphere and the Environment
University of York
-
Durham University
-
University of Oxford
-
Chemistry with Biomedicine with a Year's Professional Placement
King's College London
-
Durham University
-
Extended Degree in Biosciences and Chemistry
Sheffield Hallam University
-
Medicinal Chemistry and Biology
Keele University
We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.
- charco
- Mr M
- Changing Skies
- F1's Finest
- rayquaza17
- Notnek
- RDKGames
- davros
- Gingerbread101
- Kvothe the Arcane
- TeeEff
- The Empire Odyssey
- Protostar
- TheConfusedMedic
- nisha.sri
- claireestelle
- Doonesbury
- furryface12
- Amefish
- harryleavey
- Lemur14
- brainzistheword
- Rexar
- Sonechka
- TheAnxiousSloth
- EstelOfTheEyrie
- CoffeeAndPolitics
- an_atheist
- Labrador99
- EmilySarah00