What is the law of conservation of mass?

Hint: It is one of the basic laws that exist in nature and we encounter the uses of this law in our day to day life. This law is also known as the law of Indestructibility of matter. This law was given by Antoine Lavoisier.

Complete answer:
Let’s see the definition of this law. According to this law mass can neither be created nor be destroyed in a balanced chemical reaction or physical reaction. But one form is changed into another form. The sum of the mass of reactants reacted and the sum of the mass of products produced are equal.
If in a reaction, the reactants are not completely consumed then the relationship will be:
Total mass of the reactants = Total mass of the products + Mass of unreacted reactants
Let’s see one example,
\[{{H}_{2}}+C{{l}_{2}}\to 2HCl\]
In this reaction, the mass of hydrogen in grams in \[2g\] and the mass of chlorine in grams in \[71g\]. Therefore, the total mass of reactants is \[73g\] and if we calculate the total mass of the products, it should be \[73g\] only and indeed it comes out to be\[73g\] as the mass of \[HCl\] is \[36.5g\] but there are two moles \[HCl\], therefore total mass is \[2\times 36.5=73g\].

Note:
There is one limitation to this law. In many reactions such as nuclear reactions, The mass is converted into energy(to some extent). Therefore, energy and mass combinely is conserved in a chemical reaction.