chem help please?

Question

can someone show me how to figure out a problem like this one?

What is the empirical formula for a compound whose composition by mass is 40%Sulfur and 60%Oxygen?

I've looked all through my notes and can't find anything that will help and my book doesn't explain it well

Answer 1

Answer is SO3.

40% + 60%=100%

Assume you have 100g. of this compound.

40g. S * 1 mole S/32.07 g S= 1.25 mole S

60 g. O * 1 mole O/16.00 g O=3.75 mole O

Ok, now look to see which one has the least amount of moles.
Sulfur does at 1.25 moles.

Divide all elements # of moles with the # of moles of sulfur.

sulfur 1.25/sulfur 1.25 =1

oxygen 3.75/sulfur 1.25 =3

thus, the empirical formula is SO3.

Answer 2

Divide 40 by 32 and 60 by 16 (the relative atomic masses of the two elements concerned).

You now have two numbers which you must scale up or down (in proportion) until the smaller one equals 1. You do this by dividing the larger one by the smaller. This then leads directly to the empirical formula.