Calculate the mass of 139 cm^3 argon at stp

Volume in liters:

139 cm³ / 1000 = 0.139 L   

molar mass Argon = 39.95 g/mol

1 mole 22.4 L ( at STP )
? mole 0.139 L

moles of argon :

0.139 * 1 / 22.4 = 0.00620 moles 

Mass of argon :

0.00620 * 39.95 = 0.24769 g

hope this helps!