Question

At STP, 560 L of argon gas masses how much?

Answer 1

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)