If percentages are given, assume you have 100 g of the compound, replacing each percent symbol with "g". Sometimes, gram units are already given.
1. Convert each mass into moles.
(Mass A)(1 mol/atomic mass A) = moles A
(Mass (1 mol/atomic mass = moles B
(Mass C)(1 mol/atomic mass C) = moles C
2. Divide each mole quantity by the smallest mole quantity.
3. Look at the results from (2) and see if you can determine a whole number ratio among the set of numbers. A trick that can help you see the whole numbers is to multiply each quantity in the set by 2, 3, or 5. If none of those works, try larger numbers. Once you have a simple whole number ratio, those are the subscripts in the empirical formula.
An empirical formula of a compound uses subscripts to indicate the simplest whole-number ratio of the moles of atoms of each element to one another. For example, suppose you have the following question:
"A sample mass of nicotine contains 74.0% carbon, 8.7% hydrogen, and 17.3% nitrogen. What is the empirical formula of nicotine?
To do this problem, we first must somehow convert the percentages to masses of each element. Then we need to convert each mass to moles of atoms of each element making sure that each number of moles is a whole number, and finally using those whole numbers as subscripts in the formula C?H?N?.
Since no masses are given, let's imagine that we have exactly 100. grams of nicotine powder. Then each given percentage represents the number of grams of each respective element:
C = 74.0 g
H = 8.7 g
N = 17.3 g
To find the number of moles of each, we need to divide each of the above masses by the corresponding gram-atomic mass of the element:
C = (74.0 g) / (12.0111 g/mol) = 6.1609677 mol
H = (8.7 g) / (1.00794 g/mol) = 8.6314661 mol
N = (17.3) / (14.0067 g/mol) = 1.2351231 mol
Since the numers of moles of each element did not turn out to be whole numbers, we can divide each of the three numbers by the smallest one of the three:
Even though the first two numbers did not turn out to be exactly whole numbers, they were close enough, so it is all right to round them to the nearest whold number. In case, the numbers were not close to a whold number, it wold become necessary to multiply each of the three numbers by a small whole number such as "2" or "3" in the hope of getting a number very close to a whole number. As it is in this case, the empirical formula for nicotine turns out to be C5H7N.
Comments
That's a rather broad question. I have a number of empirical formula tutorials and solved sample problems here:
http://www.chemteam.info/Mole/Mole.html
I also took the text of your question and did a search:
https://www.google.com/search?q=how+to+do+empirica...
There might be some help for you in there.
Determining Empirical Formulas
If percentages are given, assume you have 100 g of the compound, replacing each percent symbol with "g". Sometimes, gram units are already given.
1. Convert each mass into moles.
(Mass A)(1 mol/atomic mass A) = moles A
(Mass
(1 mol/atomic mass 
= moles B
(Mass C)(1 mol/atomic mass C) = moles C
2. Divide each mole quantity by the smallest mole quantity.
3. Look at the results from (2) and see if you can determine a whole number ratio among the set of numbers. A trick that can help you see the whole numbers is to multiply each quantity in the set by 2, 3, or 5. If none of those works, try larger numbers. Once you have a simple whole number ratio, those are the subscripts in the empirical formula.
====================================================
Here's an example.
A compound is 43.7 % P and 56.3 % O by mass. Find its empirical formula.
(1) Change % to g and convert each mass into moles
P 43.7 g x 1 mol/31.0 g = 1.41 mol
O 56.3 g x 1 mol/16.0 g = 3.52 mol
(2) Divide both mol quantities by 1.41
1.41 mol / 1.41 mol = 1
3.52 mol / 1.41 mol = 2.50
(3) If you can't tell what the ratio is yet, multiply each quantity by 2
1 x 2 = 2
2.5 x 2 = 5
The empirical formula is P2O5.
An empirical formula of a compound uses subscripts to indicate the simplest whole-number ratio of the moles of atoms of each element to one another. For example, suppose you have the following question:
"A sample mass of nicotine contains 74.0% carbon, 8.7% hydrogen, and 17.3% nitrogen. What is the empirical formula of nicotine?
To do this problem, we first must somehow convert the percentages to masses of each element. Then we need to convert each mass to moles of atoms of each element making sure that each number of moles is a whole number, and finally using those whole numbers as subscripts in the formula C?H?N?.
Since no masses are given, let's imagine that we have exactly 100. grams of nicotine powder. Then each given percentage represents the number of grams of each respective element:
C = 74.0 g
H = 8.7 g
N = 17.3 g
To find the number of moles of each, we need to divide each of the above masses by the corresponding gram-atomic mass of the element:
C = (74.0 g) / (12.0111 g/mol) = 6.1609677 mol
H = (8.7 g) / (1.00794 g/mol) = 8.6314661 mol
N = (17.3) / (14.0067 g/mol) = 1.2351231 mol
Since the numers of moles of each element did not turn out to be whole numbers, we can divide each of the three numbers by the smallest one of the three:
C = (6.1609677 mol) / (1.2351231 mol) = 4.9881406 mol â 5
H = (8.6314661 mol) / (1.2351231 mol) = 6.9883448 mol â 7
N = (1.2351231 mol) / (1.2351231 mol) = 1
Even though the first two numbers did not turn out to be exactly whole numbers, they were close enough, so it is all right to round them to the nearest whold number. In case, the numbers were not close to a whold number, it wold become necessary to multiply each of the three numbers by a small whole number such as "2" or "3" in the hope of getting a number very close to a whole number. As it is in this case, the empirical formula for nicotine turns out to be C5H7N.