At stp which sample contains the same amount of molecules as 3.0 liters of h2 (g)?
1.5 l of nh3 (g)
2.0 l of co2 (g)
3.0 l of ch4 (g)
6.0 l of n2 (g)

Remember that as per the law of ideal gases the number of molecules of a gas is proportional to the volume, which means that at the same temperature and pressure, the same amount of molecules will have the same volume.

You can deduct that from pV = nRT

From there if p, and T are constants:

V = (a constant) * n

So, the same number n, will give the same number V.

Then, 3.0 molecules of H2(g) will have the same volume that 3.0 molecules of any other gas, under same pressure and temperature, assuming ideal gases.

So, the answer is the third option, 3.0 L of CH4(g).