What is the mass of 16800 mL of oxygen gas at STP

Due to having STP conditions and given volume you can use this conversion: 1 mol=22.4L at STP. Since you have mL, convert to Liters first. 16800 mL = 16.8 L. You will need molar mass of Oxygen(O2)

16.8 L O2 x(1 mol/22.4 L) x( 31.998 g O2/1 mol) = 24.0 g O2

Alternatively, maybe you did not know 1 mol =22.4 L conversion, you can use PV=nRT will work, with P = 1 atm, R =0.08206 L(atm)/mol(k), and T=273 K, V = 16.8 L. Solve for n(moles), then multiply by molar mass of O2