When u add the solution to the chemical's
The best equation which illustrate the law of conservation of mass is
Zn +2 HCl --> ZnCl2 + H2
This is because the mass of the product is equal to the mass of reactant. for example the mass of Zn is 1 in the reactant and product side. According to the law of mass conversation the mass in a isolated system is neither created nor destroyed by chemical reaction or physical transformation
It is like an idea that they come up with in an experimeant
The percent by mass of hydrogen in the compound is found by doing: (12.3/210.3) x 100 = 6%
Hope this will help