Which gas sample at stp has the same number of molecules as a 2.0-liter sample of cl2(g) at stp?
(1) 1.0 l of nh3(g) (3) 3.0 l of co2(g)
(2) 2.0 l of ch4(g) (4) 4.0 l of no(g)

(2) 2.0 L of CH4(g) is your answer.
According to Avogadro's law, all gases have equal volume if there are equal number of particles under the same conditions. Therefore, we can use the corollary to state that all gases have equal number of particles if they have the same volume, and the only choice with 2.0 L was #2, therefore #2 was your answer.