What is the mass of argon gas in a 75.0 mL volume at STP

Assuming that STP = 0°C, and 1 atm pressure:

molar volume is 22.414 L = 22 414 cm^3

75 mL = 1*75/22 414 moles = 0.0033461 moles.

mole mass = 39.9 g - - - >

0.0033461 moles = 0.0033461*39.9 g = 0.13351 g < - - - ans.