what is the law of conversation of mass

The Law of Conversation of Mass was stated by Antoine Lavoisier in 1789, when he found out that the mass of a substance cannot be created, nor destroyed in a chemical reaction. It states that the mass of an object remains the same, at any given point in time throughout any closed system. the mass of the system must remain constant overtime, as the system's mass cannot change, so any physical quantity cannot be added or removed. 

Example:

Molar Mass of Water (Chemical Formula of Water = H20) = 18.00 g/mol

H20, means two moles of hydrogen and one mole of water.

The molar mass of an element can be found in the periodic table of elements.

Molar mass of Hydrogen = 1 g/mol

(Since it is water has 2 moles of hydrogen, it means that the molar mass will be multiplied by two)

Molar mass of H2 = (1 g/mol)(2 mol)

Molar mass of H2 = 2 grams

Molar mass of Oxygen = 16 g/mol

Molar mass of H20 = 2 g + 16 g/mol

Molar mass of H20 = 18 g/mol

If we add the molar mass of H2 and O, we can see the application of the Law of Conservation of mass, since their sum equal to 18 g/mol