Law of Mass Conservation

The law of mass conservation is a fundamental principle in chemistry stating that mass is neither created nor destroyed in a chemical reaction. Put simply, the total mass of the substances that react (the reactants) must equal the total mass of the new substances that are formed (the products). The atoms are simply rearranged to form new compounds, but the total amount of matter remains the same. This principle was first articulated by Antoine Lavoisier in the late 18th century and is a cornerstone of modern chemistry.

Example Problem
Here's a common type of problem that illustrates this law:

If 10 grams of sodium (Na) react completely with chlorine (Cl) to produce 25.4 grams of sodium chloride (NaCl), how many grams of chlorine were used in the reaction?

Reactants: Sodium (Na) and Chlorine (Cl)

Product: Sodium Chloride (NaCl)

According to the law of mass conservation:
Mass of Reactants = Mass of Products

So,
Mass of Na + Mass of Cl = Mass of NaCl

We can plug in the known values:
10 g + Mass of Cl = 25.4 g

To find the mass of the chlorine, we subtract the mass of the sodium from the mass of the sodium chloride:
Mass of Cl = 25.4 g - 10 g
Mass of Cl = 15.4 g

Therefore, 15.4 grams of chlorine reacted with the 10 grams of sodium.