Avogadro's law is V1/n1 = v2/n2 - volume and number of moles are directly proportional. My question is - can this law apply to two different gases - for example, Hydrogen with volume v1 and number n1, and Oxygen with volume v2 and number n2 - are they proportional?
Since Avogadro's Law is speaking about ideal gases then it's true for all gases that behave ideal. Since ideal gases don't interact with each other and don't have any volume for single molecules (they only have a volume as a whole) Avogadro's Law holds for different gases.
Since for real gases the approximations of the ideal gas don't hold then Avogadro's Law doesn't hold. Yet a ratio similar can be achieved by dividing V over n of different gases to get the molar Volume and by using different equtions to describe real gases a ratio factor can be achieved for a given pressure and temperature.
Avogadro's Law says just that. Regardless of the gas equal volumes have equal n. (At constant p and t) and v/n is constant.
v/n=k will be true for different amounts of the same gas and approximately true between two gases.