Avogadro Hypothesis
The principle that the equal volumes of all the gases (when given the same temperature and pressure) have equal numbers of molecules .A hypothesis is a cautious explanation for the observed events. A hypothesis is not a prediction, but it should allow you to make predictions which can be verified by experiment. When results of the experiments are as predicted, it lends support to hypothesis as a good explanation, and its eventual acceptance as theory. If the results are not as predicted, the hypothesis must be modified, or replaced with a better explanation. No statement is a hypothesis unless it suggests a cause for an effect, and unless it has the possibility of being wrong. Avogadro's Law is a principle stated in the year 1811 by the Italian chemist Amedeo Avogadro that the "equal volumes of the gases at same temperature and pressure has same number of molecules regardless of their chemical nature and physical properties".
Avogadro's number is 6.022 X 1023.
It is the total number of molecules of any of the gas present in a volume of 22.41 L and is same for lightest gas as for the heavy gas like carbon dioxide or bromine. Avogadro's number is one of the fundamental constants of the chemistry. As an example, equal volumes of molecular hydrogen and nitrogen would contain the same number of molecules, as they are at the same temperature and pressure and observe ideal or perfect gas behavior. In practice, for real gases, the law only holds approximately, but the agreement is close enough for the approximation to be useful It permits calculation of the amount of pure substance (mole), the basis of stoichiometric relationships. It also makes possible determination of how much heavier a simple molecule of one gas is than that of another, as a result the relative molecular weights of gases can be ascertained by comparing the weights of equal volumes. Avogadro's number (conventionally represented by N' in chemical calculations) is now considered to be the number of atoms present in 12 grams of the carbon-12 isotope (one mole of carbon 12) and can be applied to any type of chemical entity.