[Determinator]

Hi,

I'm a CIE A level Chemistry student. There's a formula in the textbook in the chapter about mass spectrometry for calculating the number of carbon atoms in a molecule (n):

n=(100/1.1)*[(abundance of [M+1]+ ion)/(abundance of M+ ion)].

Using the binomial distribution to model this, I've derived an equation with (98.9/1.1) instead of the (100/1.1). My working is attached.

Can anybody please confirm whether I'm right and the textbook is indeed slightly wrong? Thanks!

[Borek]

My guess is that the formula is intended as an approximation, in which case making it simpler (100/1.1) doesn't make it "wrong", instead it makes it easier to remember.

[sjb]

Possibly a number of thing at play here. 100 may be simpler, there are other nuclei that will also contribute to M+1, ...

[Determinator]

Thanks for the responses guys. Checking the mark scheme, question 7(c)(i) of paper 9701/43/O/N/11, the exam indeed expects use of the textbook formula, using (100/1.1) instead of (98.9/1.1). I guess, as Borek says, the formula may be derived from experimental results.