To calculate the mass of helium in grams, we can use its molar mass. The molar mass of helium (He) is 4.00 g/mol. This means that 1 mole of helium has a mass of 4.00 grams.

Now, let's calculate the mass of 1.5 moles of helium:

Mass = Moles × Molar mass

Mass = 1.5 moles × 4.00 g/mol

Mass = 6.00 grams

Therefore, the mass of 1.5 moles of helium is 6.00 grams.