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

ch2o and c3h6o3
pbcl2 and pbcl4
no2 and n2o4
c3h12 and ch4

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)

CaBr₃ = 40 + 80 * 3 = 280 g/mol

hope this helps!