Here's how to find the empirical formulas for each molecular formula:

a) Sodium Peroxide (Na₂O₂)

* Empirical Formula: NaO

* The molecular formula already shows the simplest whole-number ratio of atoms.

b) Terephthalic Acid (C₈H₆O₄)

* Empirical Formula: C₄H₃O₂

* Divide all subscripts by the greatest common factor (2).

c) Phenobarbital (C₁₂H₁₂N₂O₃)

* Empirical Formula: C₆H₆N₂O₃

* Divide all subscripts by the greatest common factor (2).

d) 1?

* You did not provide a molecular formula for (d). Please provide the molecular formula so I can determine the empirical formula.

Remember:

* Empirical formulas represent the simplest whole-number ratio of atoms in a compound.

* Molecular formulas show the actual number of atoms of each element in a molecule.