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# Chemistry as level spectroscopy watch

1. How can you tell where the molecular ion peak is and how to you work out the molecular formula of an organic compound using its empirical formula and its mass spectrum. Also how can you tell what fragments will form and what each peak means?

2. So using this one as an example the molecular ion peak is the peak at the end of the spectrum, it is 98. Sometimes you will see a small M+1 peak which is the Mr+1 =99 ignore it.

the fragment ions will appear as large peaks they sometimes label them for you. To work out what it is, write out the structural formula so this one is C(CH3)2CHCO(CH3) then find ways to break them up, so CO(CH3)+ is the peak at 43 and C(CH3)2CH+ is responsible for the 55 peak and so on. It is handy to remember a few of by heart for eg CH3+ has Mr 15, CH3CH2+is 29

When you have the empirical formula such as C4H8 you will need to work out the Mr of that first which is (12*4)+(8)= 56. Then let us say you found the molecular ion peak to be at 112. Always divide the bigger number by the smaller number 112/56 =2 (At AS you will always get a whole number, that's one way you know if you did everything right.) So now times everything in the empirical formula by 2 so (C4H8) x 2 = C8H16

(Original post by Mf1999)
How can you tell where the molecular ion peak is and how to you work out the molecular formula of an organic compound using its empirical formula and its mass spectrum. Also how can you tell what fragments will form and what each peak means?
3. (Original post by TheAlphaParticle)

So using this one as an example the molecular ion peak is the peak at the end of the spectrum, it is 98. Sometimes you will see a small M+1 peak which is the Mr+1 =99 ignore it.

the fragment ions will appear as large peaks they sometimes label them for you. To work out what it is, write out the structural formula so this one is C(CH3)2CHCO(CH3) then find ways to break them up, so CO(CH3)+ is the peak at 43 and C(CH3)2CH+ is responsible for the 55 peak and so on. It is handy to remember a few of by heart for eg CH3+ has Mr 15, CH3CH2+is 29

When you have the empirical formula such as C4H8 you will need to work out the Mr of that first which is (12*4)+(8)= 56. Then let us say you found the molecular ion peak to be at 112. Always divide the bigger number by the smaller number 112/56 =2 (At AS you will always get a whole number, that's one way you know if you did everything right.) So now times everything in the empirical formula by 2 so (C4H8) x 2 = C8H16

Thank you so much I don't know why I couldn't understand it from my teacher she made it seem ten times more complicated

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