Yr 12 Mass spec (1 Viewer)

[hs17]

Are we meant to know how to answer this question for the HSC? Like knowing the masses of the different isotopes of bromine?

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Ans:

 

[hs17]

Hahah ok tyy! Would we need to know any other substances' isotopes e.g. Cl?

 

[CM_Tutor]

The halogens chlorine and bromine come in pairs of isotopes, a mass of two apart. You can guess what they are from the molar mass:

• chlorine comes as ³⁵Cl and ³⁷Cl, in roughly 3:1 ratio giving M = 35.5 g mol^-1
• bromine comes as ⁷⁹Br and ⁸¹Br, in roughly 1:1 ratio giving M = 79.9 g mol^-1

These are the only two that you can be expected to know, but you do need to be able to recognise the effect of other isotopes if present. For example, if you were told that one carbon atom was a ¹³C, rather than the usual ¹²C, in a sample of butanone, you would need to recognise that the molecular ion would be [¹²C₃¹³C₁H₈O]^{+ *} with m/z = 73 rather than the m/z = 72 you'd get for [¹²C₄H₈O]^{+ *}.

For a compound like dichloroethane, C₂H₄Cl₂, you'd get molecular ions at:

• m/z = 98 from [C₂H₄³⁵Cl₂]^{+ *}
• m/z = 100 from [C₂H₄³⁵Cl₁³⁷Cl₁]^{+ *}, and
• m/z = 102 from [C₂H₄³⁷Cl₂]^{+ *}

and in roughly 9:6:1 ratio (which you can calculate from a tree diagram using P(³⁵Cl) = 3/4 and P(³⁷Cl) = 1/4, so you get P(2 x ³⁵Cl) = 9/16,
P(2 x ³⁷Cl) = 1/16, and P(one of each of ³⁵Cl and ³⁵Cl) = 2 x 3/4 x 1/4 = 6/16).

 

[Momentazeus]

If you don't mind me asking, what module is this?

 

[Life'sHard]

If you don't mind me asking, what module is this?

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