Let's assume that the gas is an ideal gas.
1 mole of ideal gas has the volume as 22.4 L at
STP (273 K , 1 atm)
The given volume is 115.6 L.
moles in 22.4 L of gas = 1 mol
moles in 115.6 L of gas = (1 mol x 115.6 L) / 22.4 L
= 5.16 mol