Reaction of current interest is:
CS2(l) + Cl2(g) → CCl4(l) + S2Cl2(l)
While balancing the chemical reaction, care must be taken that number of atoms of reactant side is equal to number of reactant on product side.
In present case, There is 1 'C' atom of both reactant and product side
There are 2 'S' atoms on both reactant and product side.
However, there are 2 Cl atoms of reactant side, but 6 Cl atoms on product side.
Hence multiplying Cl2 by 3, would equal the number of Cl atoms on both the sides.
Thus, the balanced reaction is
CS2(l) + 3 Cl2(g) → CCl4(l) + S2Cl2(l)
