What is the empirical formula of a substance composed of 3.05% carbon,
0.26% hydrogen, and 96.69% iodine?
a. CHI3
b. C2HI7
c. CH2I5
C3H2I11

To answer this question, you need to know the molecular weight of the
involved elements.

H - 1
C - 12
I - 127

The periodic table, along with many good tutorials on chemistry can be
found here:
http://www.webelements.com/

The weights of the answers are:
396   with H = 1  C = 12  I = 381
914        H = 1  C = 24  I = 889
649        H = 2  C = 12  I = 635
1435       H = 2  C = 36  I = 1397

Picking H, the percentages in the answers are:
0.25%
0.11%
0.30%
0.13%

So, the only two that are close are the first and the third, moving to
carbon, it's percentages are:
3%
1.8%

So, the first answer matches, for completeness, calculating the
percentage of I gives:
96%


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