Explanation:
According to Avogadro's law, equal volumes of all the gases at same temperature and pressure will also, have same number of molecules.
That is, 
or
= k
Since, it is given that
is 6.13 L,
is 8.51 mol, and
is 18.5 L.
Hence, calculate the
as follows.

= 0.35 mol
Thus, we can conclude that the number of moles of gas added to the container is 0.35 mol.