Example: If a compound is 47.3% C (carbon), 10.6% H (hydrogen) and 42.0% S (sulfur), what is its empirical formula?
To do this problem we need to transfer all of our percents to masses. We assume that we have 100 g of this substance. Then we convert to moles:
Carbon:
47.3 grams
1
x
1 mole
12.01 grams
= 3.94 moles
Hyrdrogen:
10.6 grams
1
x
1 mole
1.008 grams
= 10.52 moles
Sulfur:
42.0 grams
1
x
1 mole
32.07 grams
= 1.310 moles
Now we try to get an even ratio between the elements so we divide by the number of moles of sulfur, because it is the smallest number:
Carbon:
3.94
1.310
= 3
Hydrogen:
10.52
1.310
= 8
Sulfur:
1.310
1.310
= 1
So we have: C3H8 S