Final answer:
The law illustrated by the given observation is the Law of Definite Proportions, which states that a chemical compound always has the same proportion of elements by mass, making this a chemistry concept applicable at the high school level.
Step-by-step explanation:
The observation that 20g of hydrogen gas always combines with 160g of oxygen gas to form 180g of water, even when there is more than 160g of oxygen present in the reaction container, illustrates the Law of Definite Proportions. This law states that a given chemical compound always contains exactly the same proportion of elements by mass. An example of this law is seen in water, where every sample of pure water contains 11.19% hydrogen and 88.81% oxygen by mass, irrespective of the water source or how the water was prepared.
The Law of Definite Proportions can only be applied to situations where the same product is being formed, such as when hydrogen and oxygen combine to form water. It would not be applicable if hydrogen and oxygen were to combine to form a different compound, like hydrogen peroxide. Furthermore, the Law of Conservation of Mass is also evident here, which indicates that the total mass of the reactants equals the total mass of the products.