How can you prove the Conservation of Mass? use 2Li + F2 -> 2LiF as an Example

-First find the Total of the Relative Formula Mass of the Reactants 2Li= 2 x 6.9= 13.8. F2 = 2x19=38. 38+13.8=51.8 -Then find the Total of the Relative Formula Mass of the Product 2liF= [13.8x2]+[19x2] = 51.8 -Answers are the same, no Mass is Lost or Gained