What is an example of an Avogadro's law practice problem?

As a consequence of Avogadro's law, different gases in same conditions have the same number of molecules in the same volume.

But, you can't see molecules. So, how can you ascertain the law? The "sameness" of particle number?

The answer is: through experiments based on the different weight of the different gases. Yes! in fact air and other gases have a weight, because they are made of particles.

A same number of heavier molecules have a greater weight, while an equal number of lighter molecules have a lower weight.

Examples
I. Where does moist air go? Upward. Because it contains more water molecules ( #H_2O#, mass = 16+1+1 = 18) and these are lighter of oxygen (#O_2#, mass = 16+16 = 32) and nitrogen (#N_2# mass = 14+14 = 28). It is known that humidity rises uphill, especially in fall.

II. A balloon full of Hydrogen or Helium gas is lighter than air, so it will raise in air. Avogadro's law can make you fly.

III. A soup bubble of air is lighter than the same volume of #CO_2# (carbon dioxyde) as you can see from this video:

IV. A beaker full of heavy molecules of #CO_2# can be tilted upon a flame. The gas will displace the air and will estinguish the flame.

V. A liter of sulfur hexafluoride has the same weigt of 5 liters of air (because its molecules are heavier in the same ratio of the average air molecule). Consequently, a light bowl full of air will float upon a bath of #SF_6#, as shown in this video.

VI. Once you have had enough fun, you can try a practice problem about Avogadro's law, as the following.

Given that one liter of hydrogen weighs 0.0836 grams at 20 celsius degrees, while a liter of Helium, at the same temperature, weighs 0.167 grams, exactly the double. Yet, helium atoms are four times heavier than hydrogen atoms, and not the double. So, how can you explain why a liter of helium is only the double heavier of a liter of hydrogen, instead that 4 times heavier?"
Solution . Hydrogen gas i formed of "diatomic" molecules (#H_2#) while helium is "monoatomic", (He). Therefore, in the liter of hydrogen there are the same number of (#H_2#) molecules as many He atoms are in the liter of helium, and a helium atom weighs as two hydrogen atoms:

#1He# = #4H# (in mass)
#1He# = #2H_2# (in mass)
1 liter of #He(g)# has double mass of 1 liter of #H_2(g)#