Final answer:
The mass in grams of 120 liters of He gas at STP is 5.4 g.
Step-by-step explanation:
To find the mass of 120 liters of He gas at STP, we need to use the molar volume of the gas at STP. The molar volume of any gas at STP is 22.4 liters/mol. So, we can set up the following conversion:
120 L He gas × (1 mol He gas/22.4 L He gas) × (4.0 g He gas/1 mol He gas) = 5.4 g He gas
Therefore, the mass in grams of 120 liters of He gas at STP is 5.4 g.