i) #Na + Cl_2 = NaCl#
Here, two #Cl# s are in the left hand side of the equation, but one on the right. So, mass is not conserved.
ii) #2H_2O_2 = 2H_2O + O_2#
Here 2 #H# and 4 #O#'s are present on the left hand side, and same on the right side.
So, mass is conserved.
iii) #CH_4 + O_2 = CO_2 + 2H_2O#
Here 1 #C#, 2 #O#, and 4 #H# are present on the left hand side, and same in the right side of the equation too.
So, Mass is conserved.
iv) #AgNO_3 + MgCl_2 = AgCl + Mg(NO_3)_2#
Here, #Ag# and #Mg# are balanced, but #Cl# and #NO_3# are not.
Mass is not conserved.
