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mass spectrometer

please may someone help me with this? I can't understand what I'm supposed to do for the calculation :frown:

the mass spectrum of a gallium sample contains only 2 peaks at mass/charge ratios 69 and 71
the heights of these peaks in a mass spectrum of gallium are 145mm and 95mm
calculate the relative abundance of each isotope of gallium to one decimal place

calculate the relative atomic mass of gallium to two decimal places

why is it a good approximation to assume the relative mass of the Ga+ cations, calculated from mass spectrometry, is the same as the relative atomic mass of Ga?
(edited 9 years ago)
Original post by bceg12
the mass spectrum of a gallium sample contains only 2 peaks at mass/charge ratios 69 and 71
the heights of these peaks in a mass spectrum of gallium are 145mm and 95mm
calculate the relative abundance of each isotope of gallium to one decimal place

calculate the relative atomic mass of gallium to two decimal places

why is it a good approximation to assume the relative mass of the Ga+ cations, calculated from mass spectrometry, is the same as the relative atomic mass of Ga?


The height of the peaks is proportional to the relative abundance of the isotopes. Hence if (145 + 95) = 100% then you can work out the relative abundance of each.
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Avatar for bceg12
bceg12
OP
Original post by charco
The height of the peaks is proportional to the relative abundance of the isotopes. Hence if (145 + 95) = 100% then you can work out the relative abundance of each.


Sorry but I really don't understand, I think it is the wording of the question :frown: These are the only three out of 25 that I couldn't get
Original post by bceg12
Sorry but I really don't understand, I think it is the wording of the question :frown: These are the only three out of 25 that I couldn't get


Which part of the sentence don't you understand?
Reply 4
Avatar for GDN
GDN
1
when calculating relative atomic mass from mass spectrum data the relative abundances of the different isotopes do not necessarily have to be quoted as percentages - in this problem the sum of the heights of the peaks represents the total amount of the 2 isotopes so the height of each peak divided by the sum of the heights of the 2 peaks gives the fraction of each isotope
Reply 5
Avatar for Borek
Borek
4
Gallium has two isotopes. Let's say fraction of the first is x1 and fraction of the second is x2.

You are told their relative abundances are

x1x2=14595\frac{x_1}{x_2} = \frac{145}{95}

For obvious reasons these fractions sum up to 1:

x1+x2=1x_1 + x_2 = 1

Can you calculate now their percent abundances?

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