isotopes and mass spectrums

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[roseybabyy]

I'm having trouble with this problem. I don't even know where to start.

"Gallium arsenide, GaAs, has gained widespread use in semiconductor devices that convert light and electrical signals in fiberoptic communications systems. Gallium consists of 60% ⁶⁹Ga and 40% ⁷¹Ga. Arsenic has only one naturally occurring isotope, ⁷⁵As. Gallium arsenide is a polymeric material, but its mass spectrum shows fragments with the formulas GaAs and Ga₂As₂. What would the distribution of peaks look like for these two fragments?"

HELP!!

[Otis]

Wow, Lets help out.

Given your information you can only have two possible product with regard to isotopes ⁶⁹Ga⁷⁵As and ⁷¹Ga⁷⁵As. You would have peaks at 144 and 146 with a relative size of 6 to 4 respectively (just like the percentages)

Otis

[valdorod]

Wow, Lets help out.

Given your information you can only have two possible product with regard to isotopes ⁶⁹Ga⁷⁵As and ⁷¹Ga⁷⁵As. You would have peaks at 144 and 146 with a relative size of 6 to 4 respectively (just like the percentages)

Otis

For this case the spectrum should look like this


For the second fragment the Ga₂As₂ you have two choices for Ga and one choice for As

This gives us the following posibilities
⁶⁹Ga₂⁷¹As₂ ,⁶⁹Ga⁷¹Ga⁷¹As₂ , and ⁷¹Ga₂⁷¹As₂ resulting in a triplet. 
 
First assuming that the first Ga is Ga-69, a 60% chance.  The second Ga can either be Ga-69 a 60% chance or Ga-71 40% chance

The probabilities are
 
Ga-69 and Ga-69 = 0.6 x 0.6 = 0.36
Ga-69 and Ga-71 = 0.6 x 0.4 = 0.24

Second we assume that the first Ga is Ga-71 then we have

Ga-71 and Ga-69 = 0.4 x 0.6 = 0.24
Ga-71 and Ga-71 = 0.4 x 0.4 = 0.16

two of the choices above are the same, so we add their probabilities and we have

⁶⁹Ga₂⁷¹As₂ ,= 0.36

⁶⁹Ga⁷¹Ga⁷¹As₂ or ⁷¹Ga⁶⁹Ga⁷¹As₂ = 0.24 + 0.24 = 0.48

⁷¹Ga₂⁷¹As₂ = 0.16

So the triplet has the smallest peak at the highest mass (⁷¹Ga₂⁷¹As₂)
the largest peak at the middle mass (⁶⁹Ga⁷¹Ga⁷¹As₂ or ⁷¹Ga⁶⁹Ga⁷¹As₂)

the lightest species,⁶⁹Ga₂⁷¹As₂, has a peak intermediate in height.

and would look something like this

 

[Otis]

Oops, I missed the Ga₂As₂.

Otis