Question

Which equation shows a conservation of mass?
(1) Na + Cl2 --> NaCl
(2) Al + Br2 --> AlBr3
(3) H2O --> H + O2
(4) PCl5 --> PCl3 + Cl2

Answer 1

(4) PCl5--> PCl3+ Cl2 is the correct answer because we both have 1 P atom and 5 Cl atoms on both sides, and thus the masses of the elements don't change.
Hope this would help~