Question

Find the natural abundance of Si-30.

Answer 1

this is more math than anything...

First you need to know how many isotopes there are of silicon, and it's average atomic units (look at periodic table). Then make up a system of equations to solve for it. There's 3 stable silicon isotopes (28, 29, 30) so you will need to have 3 equations. You must be given the percent abundance of at least one of the isotopes to solve because here I can only see 2 equations (numbered down below)

set x = percent abundance of si-28
y = percent abundance of si-29
z = percent abundance of si-30

since all of silicon atoms account for 100% of all silicon:
x + y + z = 100% = 1
therefore:

1) x = 1 - y - z


You also have
2) 28x + 29y + 30z = average atomic mass
you can substitute x so that equation becomes:

28 (1 - y - z) + 29y + 30z = average atomic mass

See how you have 2 variables here? You can't go on until you know the value of one isotope already or you have given a clue which you can derive the third equation