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# Chemistry exams watch

1. A sample of antimony Ar= 121.7 was analyses and was found to consist of 60% 121 Sb and one other isotope. Determine the mass number of the other isotope in the sample of antimony

I know the answer is Sb123 but idk why so please will someone work through it for me x

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2. (Original post by halliethestudent)
A sample of antimony Ar= 121.7 was analyses and was found to consist of 60% 121 Sb and one other isotope. Determine the mass number of the other isotope in the sample of antimony

I know the answer is Sb123 but idk why so please will someone work through it for me x

Posted from TSR Mobile
Relative atomic mass = [(mass1 x abundance1)+(mass2 x abundance2)]/ total abundance

So if x is the 2nd isotope's mass...and the abundance is out of 100%, and one of the isotopes is 60%, so the other must be 100-60=40%...

121.7=[(121 * 60%) + (x * 40%)]/100
You need to rearrange that to make X the subject.
Spoiler:
Show
X=[121.7*100-(121*60)]/40

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