Question

23 grams of sodium reacted with 35.5 grams of chlorine. Calculate the mass of the sodium chloride compound formed

Answer 1

According to law of conservation of mass within a reaction,
The mass of the compound formed is (23+35.5) grams means 58.5 grams of sodium chloride[NaCl] will be formed.

Answer 2

Law of Conservation of Mass:
Mass is neither Created nor Destroyed.
So basically, the mass of any element (the example here is Sodium Chloride) at the start is = to the mass of the element at the finishing point. 
We add the mass:
23+35.5=58.5g
In conclusion, the mass of Sodium Chloride formed is 58.5g
Its as simple as that!