How can I find emperical formulas from just precent compositions??

Question

example: If I had a compound with 10.4% carbon, 27.8% sulfur, and 61.7% calcium. How can I find out its formula??

Answer 1

Here is what I get:

C.ratio = 10.4/12.011 = 0.866
S.ratio = m27.8/32.06 = 0.867
Ca.ratio = 61.7/40.08 = 1.539

Divide all ratios by 0.866.

C.rationorm=0.866/0.866 = 1.000
S.rationorm = 0.866/0.867 = 0.999
Ca.rationorm = 1.539/0.866 = 1.776


Now we need to turn all of these into integers. Multiply all by 9. I found 9 by trying numbers until all ratios were nearly integers.

C.rationorm=9
S.rationorm = 9 = 8.99
Ca.rationorm = 15.98

A reasonable empirical formula would be C9S9Ca16.

Answer 2

Simple - just replace the % signs by grams. Then divide by the Ar values as normal.

Answer 3

You divide each % by the molecular weight of each element:
for example in your compound you will have
10.4/12.011 atoms of C
27.8/32.066 atoms of S
61.7/40.078 atoms of Ca