Define stoichiometry. What is the etymological meaning of stoichiometry in Greek? And what is stochiometric compounds?

Stoichiometry is defined as the branch of chemistry that deals with the mass-volume relationship of elements in a chemical change.

The etymological meaning of stoichiometry is measurement of elements. In Greek, stoicheion means element and metron means to measure. Stoichiometry = Stoicheion + Metron

Those substances that obey the laws of Stoichiometry are called stoichiometric compounds.

Phosphorous and oxygen combined to each other to give PO and PO. Which law of stoichiometry is illustrated. Also state the law.

Here,
In P₂O₃, 62gm of Phosphorous combined with 48gm of oxygen.
In P₂O₅, 62gm of Phosphorous combined with 80gm of oxygen.

Ratio of mass of oxygen in P₂O₃ and P₂O₅ = 48:80 = 3:5 which is simple whole number ratio.

In this reaction, the ratio of mass of oxygen in PO and PO combined with fixed mass of phosphorous bear simple whole number ratio.

This illustrates, the law of multiple proportion of stoichiometry which states that, "When two elements combined to each other to give...

12g of Carbon combines with 32g of oxygen to give 44g of carbon dioxide. Which law of stoichiometry is illustrated? And also state the law.

Ans: 
In this reaction, the total mass of reactants is equal to the total mass of the product.

This illustrates the law of conservation of mass which states, "Under similar conditions of temperature and pressure, the total mass of reactants in any chemical reaction is equal to the total mass of product."