Moles calculation
Hi,
Please could I have help on the question below because I’m really confused which of my methods are right because they are giving different answers? So I started by doing moles=mass/mr so I got 2 moles of Sodium sulfate. Then it asked for ions so I did avorgados constant multiplied by 2 mol multiplied by 3 because there are 3 moles of ions ( 2 sodium ions and one sulfate ion). But I don’t know if it’s correct to multiply by 7 too because could you also say there are 7 ions ( 2 sodium one sulfide and 4 oxide ions)? They both give different answers though? Thank you very much!
Please could I have help on the question below because I’m really confused which of my methods are right because they are giving different answers? So I started by doing moles=mass/mr so I got 2 moles of Sodium sulfate. Then it asked for ions so I did avorgados constant multiplied by 2 mol multiplied by 3 because there are 3 moles of ions ( 2 sodium ions and one sulfate ion). But I don’t know if it’s correct to multiply by 7 too because could you also say there are 7 ions ( 2 sodium one sulfide and 4 oxide ions)? They both give different answers though? Thank you very much!
(edited 7 months ago)
The first way you did it is the correct one. Sodium sulphate is composed of two sodium ions and one sulphate ion per molecule - three ions total. The atoms within the sulphate group are covalently bonded, so none of the atoms within the sulphate are actually ions. Hope this helps 
Original post by Scotland Yard
The first way you did it is the correct one. Sodium sulphate is composed of two sodium ions and one sulphate ion per molecule - three ions total. The atoms within the sulphate group are covalently bonded, so none of the atoms within the sulphate are actually ions. Hope this helps
Thank you for clarifying. In that case, how can I do the question below because both option a and b have 5 moles of ions so how could I work out which one has the largest number? Sorry for asking so many questions but thank you a lot!!
Original post by Ashirs
Thank you for clarifying. In that case, how can I do the question below because both option a and b have 5 moles of ions so how could I work out which one has the largest number? Sorry for asking so many questions but thank you a lot!!
BUT option C has 2 x 4 ...
and in maths
(1 x 5) is less than (2 x 4)
Erm...
You've done the same mistake you were thinking of doing with your first question, to assume that every atom within the molecule is an ion, when it isn't. For A, you think the molecule has 17 ions when it only has 2 Al ion and 3 SO4 ions, so 5 total. Similar mistakes for the rest but I see you've annotated the actual ions, so I think you've realised this mistake. And you don't need the Avogadro constant for this, you can just easily calculate the number of moles of ions by figuring out the number of ions in each molecule and then multiply that by the number of moles of said molecule and the one with the biggest number of moles will be the correct answer - as charco points out, that's option C
You've done the same mistake you were thinking of doing with your first question, to assume that every atom within the molecule is an ion, when it isn't. For A, you think the molecule has 17 ions when it only has 2 Al ion and 3 SO4 ions, so 5 total. Similar mistakes for the rest but I see you've annotated the actual ions, so I think you've realised this mistake. And you don't need the Avogadro constant for this, you can just easily calculate the number of moles of ions by figuring out the number of ions in each molecule and then multiply that by the number of moles of said molecule and the one with the biggest number of moles will be the correct answer - as charco points out, that's option C
(edited 7 months ago)
Original post by charco
BUT option C has 2 x 4 ...
and in maths
(1 x 5) is less than (2 x 4)
and in maths
(1 x 5) is less than (2 x 4)
Yes i don’t why I completely missed that but c does seem correct although my teacher said a was the correct answer? I’ll ask her again anyway. Thank you!!
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