Chemistry
posted by Mae .
It is possible, although less accurate, to do a standard addition with only one sample. An unknown Fe sample was analyzed by AAS and gave an absorbance reading of 0.160. To the same sample we added 1.5ppm Fe and the new absorbance reading was 0.205. Calculate the ppm of the unknown Fe.
Could someone please explain to me on how to do this question step by step. Thanks.

You could probably set up a proportion.
so....
0.160/x= 0.205/1.5ppm
Solve for x. 
I agree it probably can be done with a proportion; however, I wouldn't set up the proportion that way.
0.160 is for x
0.205 is not for 1.5 ppm but for 1.5 ppm+xppm.
(0.160/x) = (0.205/1.5+x) and solve for x.
Another way.
0.205 = 1.5 + x
0.160 = ......x

0.045absorbance due to 1.5 ppm
Then 1.5 ppm x 0.160/0.045 = ?