Chemistry
posted by Soonjung .
Assume that the element 107Uns is synthesized and that the sample contains 39.8% 270Uns (269.4 amu), 35.2% 271Uns (270.3 amu) and 25.0% 272Uns (271.2 amu). What is the value of the atomic weight?
A. 270.
B. 269.3
C. 270.4
D. 271

So this is what I did, but for some reason, it continues to not turn out. SO i am thinking is it significant figures reason?
here is what i have, please tell me what i am doing wrong and what the answer is?
this is what i did:
(39.8/100) (269.4 amu) + (35.2/100) (270.3 amu) + (25.0/100)(271.2 amu)
And I got 270.1668 amu
But it isn't one of the choices. So I was thinking is it because of significant figures or miscalculation? OR what would the answer be?

It can't be B and the mass is less than the mass of any of the isotopes.
The % values are to three sig figs so might expect the answer to be to 3 sig figs. The only one that fits is A.
If you round each step to 3 sig figs and then add you get 269.9 or 270 to 3 sig figs.