Question

With the help of an activity, prove the law of conservation of mass....it's urgent!!!!

Answer 1

Law of conservation of mass states that If one or more substance undergo a reaction, then the mass of reactants and the products remains same. 

Example:- 

CaO + CO2 ------------> CaCO3

Here mass of CaO is 56 amu and mass of CO2 is 44 amu, and mass of CaCO3 is 100 amu . 
This shows that, 
mass of reactants= mass of products 
56 + 44= 100

i hope this is useful

Answer 2

If we will take AgNo3+NaCl it will give rise to AgCl+NaNo3
The mass is conserved both sides
Which proves the law of conservation of mass
For more details u can check this photo