# Can someone please work this out for me?

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#1
This question is a practice question in my notes, and I'm a bit shaky when it comes to calculations so I just wanted to check that the answer I have (62.96%) is correct.

"Calculate the percentage by mass of water of crystallisation in washing soda, sodium carbonate-10-water."

Thanks!
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#2
I've also stumbled upon a question asking how many atoms are in 3.55g of chlorine atoms - the number of moles in 3.55g of chlorine atoms is 0.1 (I think!), but how would I go from here to work out the number of atoms? When I did this question in my booklet, I divided the Avogadro constant by 10 to get my answer, but I read something online about multiplying the number of moles by the Avogadro constant. That method gives me the correct answer in this instance, but can it be applied to any question which asks for the number of atoms (assuming I already have the number of moles)?

These questions probably seem a bit silly, but I'm still quite new to chemistry so parts of it boggle my mind!
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7 years ago
#3
You're answer to the percentage by mass is correct, although I get 62.94% but that may be due to the atomic masses I used
And in answer to your second question, you always multiply by Avogradro's number to find the number of atoms when you have the number of moles

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#4
(Original post by BlossomS)
You're answer to the percentage by mass is correct, although I get 62.94% but that may be due to the atomic masses I used
And in answer to your second question, you always multiply by Avogradro's number to find the number of atoms when you have the number of moles

Posted from TSR Mobile
Thank you very much for your help!
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7 years ago
#5
Think about mole just as if it was an overgrow dozen. If you are asked "how many balls in half a dozen" the answer is obvious - 0.5*12. When you are asked "how many molecules in half a mole of water" it is exactly the same, just instead of 12 (number of objects in a dozen) you use NA (number of objects in a mole) - so it is 0.5*6.02x1023.
2
7 years ago
#6
(Original post by la95)
Thank you very much for your help!
You're most welcome! Good luck!
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#7
(Original post by Borek)
Think about mole just as if it was an overgrow dozen. If you are asked "how many balls in half a dozen" the answer is obvious - 0.5*12. When you are asked "how many molecules in half a mole of water" it is exactly the same, just instead of 12 (number of objects in a dozen) you use NA (number of objects in a mole) - so it is 0.5*6.02x1023.
Thanks for your help! That's a really good way of explaining it!
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#8
(Original post by BlossomS)
You're most welcome! Good luck!
I take it you studied A Level Chemistry?
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7 years ago
#9
(Original post by la95)
I take it you studied A Level Chemistry?
I did and I'm doing my second year of chemistry now (in Year 13) Are you an AS student?
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#10
(Original post by BlossomS)
I did and I'm doing my second year of chemistry now (in Year 13) Are you an AS student?
Ahh fantastic, how did you do at AS? Sort of - I already have three A Levels, but now I'm doing four further AS Levels (including chemistry)!
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7 years ago
#11
(Original post by la95)
Ahh fantastic, how did you do at AS? Sort of - I already have three A Levels, but now I'm doing four further AS Levels (including chemistry)!
I was a few marks off an A because of poor practical briefing by teacher Oh wow - what are those A levels? And why are you doing AS again?
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