Empirical formulas are the combination of joining atoms in the simplest ratio
For Al2Br6 it would be AlBr3
For Na2S2O4 it would be Na2SO4 ( I think you meant to show it like this Na2(SO4)2
For N2O5 it would be N2O5 since it is already in the simplest ratio
For K2CR2O7 it would be K2Cr2O7( It is in fixed ratio since its already in the fixed ratio)
Edit: Thanks to the other answerer who pointed out my mistake. For a second i was confused by potassium chromate and this formula. It is already in the molecular formula so those are the fixed ratio hence empirical formula applies the same .
Empirical formula is the simplest whole number ratio of atoms in a compound. What you have is called molecular formula.
We have Al2Br6. See how 2 and 6 can both be divided by 2? 2/2=1 and 6/2=3. So as an answer we have…
Do the same thing for the second one, but the 3rd and 4th are different. N205 is already in its empirical formula. So the empirical formula for the third one is the same as what’s given to you: N2O5
EDIT: Chris J is incorrect on the 4th one. The molecule is already in it’s empirical formula. It’s Potassium Dichromate.
The second one is wrong. It is NaSO2.