Final answer:
No, you do not need to know the actual mass of a substance to determine its empirical formula, because you can use percent composition and assume a convenient mass to calculate the empirical formula.
Step-by-step explanation:
You do not need to know the actual mass of a substance when determining the empirical formula using percent composition. The percent composition can be used to calculate the masses of elements present in any arbitrary, convenient mass of compound, often a 100-g sample.
These calculated masses can be converted into moles using the molar masses of the elements. Finally, the mole ratios are simplified to their lowest whole numbers to determine the empirical formula.
For example, if you have the percent composition of carbon and hydrogen in a compound, and it's 40% carbon and 60% hydrogen, you could assume a 100 g sample, which would mean you have 40 g of carbon and 60 g of hydrogen.
Then, using the molar masses of carbon (12.01 g/mol) and hydrogen (1.008 g/mol), convert these masses to moles; thus, 40 g of carbon is 3.33 moles (40 g / 12.01 g/mol) and 60 g of hydrogen is 59.52 moles (60 g / 1.008 g/mol). You then simplify the mole ratio to get the empirical formula.