Question

How do you find the empirical formula of a compound when you are given its mass in moles?

Answer 1

set the element with lowest moles equal to 1 then find the ratio..

eg. H= 0.50 moles
O= 0.25 moles

O =1 .. H = 0.50/0.25 = 2

thus empirical formula H2O

Answer 2

Moles cannot be used to measure mass. A mole is an amount.

Still, aaronq is right that if you are given the *amount* (not mass) in moles of certain elements in a compound, their ratios will give you the empirical formula.