The gas that makes up about 12 percent of the earths atmosphere is ?

You may be referring to the gas that makes up 21% of the earth's atmosphere, which is oxygen.

Explanation:

According to NASA, the gases in Earth's atmosphere include:

Nitrogen — 78 percent

Oxygen — 21 percent

Argon — 0.93 percent

Carbon dioxide — 0.04 percent

(Trace amounts of neon, helium, methane, krypton and hydrogen, as well as water vapor)

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.