It is known that at if a gas is identified to be as ideal gas then, at STP, the volume of 1 mole of that gas is equal to 22.4 L. Using this fact and the given volume in this item, we determine the number of moles as that below.
n = (11.2 L of argon)(1 mol argon / 22.4 L of argon)
n = 0.5 moles
Answer: 0.5 moles