Question

Which of the following pairs does not share the same empirical formula?

Answer 1

The correct answer is $PbCl_2$ and $PbCl_4$.  If you divide all atoms of $PbCl_4$ by factor 4 it gives $PbCl$ which is not the same as $PbCl_2$.

If you divide all atoms of $C_3H_6O_3$ by 3 you get the other pair $CH_2O$

If you divide all atoms of $N_2O_4$ by 4 you get the other pair $NO_2$

and if you divide all atoms of $C_3H_1_2$ by 3 you get the other pair $CH_4$

Answer 2

PbCl2 and PbCl4

Empirical formula is when the molecular formula is reduced (each element) by the same number until you are unable to do so any further.
CH2O cannot be further reduced, but C3H6O3 can all be divided by 3 making it CH2O (same empirical formula)