4. State the law of conservation of mass. How is it verified experimentally?

The law of conservation of mass states that during in a physical or chemical change, the mass of products remains equal to the total of reactants.  

For example: When 100g of mercuric oxide is heated, 92.6 g of mercury and 7.4 g of oxygen are obtained as products. i.e. HgO → Hg + O2

In this case mass of reactant =100g  

Mass of products = Mass of Hg + Mass of O2= 92.6 + 7.4 = 100g  

Thus, we obtain Mass of product = Mass of reactants. This verifies the law of conservation of mass.

Explanation:

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.

i think its 100.

hope this !