At STP, 560 L of argon gas masses how much?
One mole of Argon weighs 39.948g
One litre of any gas at STP has a volume of 22.4 L
So #(540L)/(22.4L) = 25 moles of argon
25 moles Ar x 39.948 g/mol = 998.7 g
Since you started with a measurement containing only two significant digits, your answer can only contain two significant digits, so it rounds to 1.0x10^3 g (or 1.0 kg)