Help with Isotopes!!!?

Question

Silicon has 3 isotopes, Si-28, Si-29, and Si-32.
If the mass (amu) of Si-28 is 27.98 with abundance of 92.21% and the mass of Si-32 is 29.97 3.09%, what is the mass of Si-29 with an abundance of 4.70%?
i dont know if abundance has anything to do with finding the mass, but just put it.
do i have to find the average mass (add the masses of Si-28 and 32 together and divide by 2?)
help please!!!! :) THANKS!

i think my book made a mistake, because it says that th mass of Si-32 is 29.97, when that is the mass of Si-30. you guys are right, because the mass should be about 31.97.
So Si-29 should be about 28.97.
THANKS!!! =)

Answer 1

It does have to do, due to the standard deviation, that means that when finding an average, you most know the values and the frecuency of those values. Anyway

total mass average = isotope1*abundance1 + isotope2*aundance2... and so on
Therefore
28,0855 = 27,98*0,9221 + 29,97*0,0309 + ?*0,0470
28,0855 = 25,74 + 0,926 + ?*0,0470
1,42 = ?*0,0470
30,2 uma = ?
Si-29 has a mass of 30,2amu

Answer 2

An abundance of 92.21% just means that 92.21% of the silicon found in nature is Si-28.

To find the average mass of silicon, you would multiply each percentage by it's respective isotope's mass. In this case:

27.98*.9221 + .0470* (mass of Si-29) + 29.97*.0309 = Whatever the periodic table says is the atomic mass of silicon. Then just solve the algebra.

However, it seems VERY odd to me that the mass of Si-32 is only 29.97, since it should be about 32(isotopes being named after how many protons/neutrons they have, which have an atomic mass of approximately 1). I would double check that.

Answer 3

Mr(total)=Mr1*%si-28+Mr2*%si-29+Mr3*%si-32/100

Answer 4

Hi. Good to see you thought about it before asking! I think if you have the average mass of silicon (28.086) then you can subtract the two known isotope masses on a percentage basis. Anyone?

Answer 5

=Study and find out